My son is 9 years old and lately he just comes home from school, throws his bag down, and won’t talk to anyone. Yesterday he snapped at his little sister for just saying hi to him. No big fight at school that I know of, no bad grades, nothing obvious going on. He used to be such a chatty kid. I try asking him what’s wrong and he just says “nothing” and walks away. I don’t want to push too hard and make things worse but sitting back and doing nothing feels wrong too. Has anyone been through this? What do you actually do when your kid goes moody for no clear reason? I feel like I’m walking on eggshells at home and I don’t even know why 
Oh man, I remember this phase so well. My daughter did the exact same thing around age 8 or 9. Came home one day and just… shut down. We thought something serious happened at school. Turns out she was just overwhelmed with a friendship thing that she didn’t even know how to explain yet. Kids at that age are processing so much more emotion than they can actually put into words. The “nothing is wrong” answer is usually not them being difficult, it’s them genuinely not knowing how to say what they feel. What worked for us was doing something side by side, like drawing or playing a simple game, and just not asking directly. She opened up after about 20 minutes of just being together without pressure. Give it time, don’t force the conversation 
Bro just wait it out. kids go through moods like the weather changes. One day it’s sunshine, next day it’s a full thunderstorm with zero warning. My nephew is 10 and my sister called me panicking one time because he wouldn’t eat dinner and was crying for no reason. Three days later he was back to being loud and annoying like nothing happened. Sometimes there is no deep reason. Their little brains are just recalibrating. That said, if it goes on for more than a week or two consistently, maybe worth a closer look. But one moody afternoon? That’s just Tuesday lol 
I’d take this a bit more seriously than Fluxorix is suggesting, no offense to him. Sudden mood shifts in kids that age can sometimes be a sign that something is bothering them that they are too embarrassed or scared to bring up directly. It doesn’t have to be anything major, it could be a friend situation, something a teacher said, or even just feeling left out at recess. I work with kids in an after school program and you’d be surprised how something that seems tiny to an adult feels enormous to a 9 year old. Keep the door open. Tell him directly that he doesn’t have to talk right now but you’re there when he’s ready. That simple sentence does a lot more than you think.
Silicrypte is right about keeping the door open. Also, one thing I started doing with my kids is what I call the “car conversation” trick. Something about being in the car where there’s no eye contact makes kids talk WAY more freely. I’d pick them up, put on some low music, and just say “anything good happen today?” not “what’s wrong” or “why are you upset.” Framing it positively makes them less defensive. My son who used to go silent on me started chatting in the car almost every day once I stopped asking loaded questions. It sounds small but the setting and the wording genuinely changes the whole dynamic.
Can we also talk about how exhausting parenting is in these moments.. Like you’ve had your own whole day, you’re tired, and now you’ve got a tiny human giving you the silent treatment with the energy of a disappointed CEO. Meanwhile you’re just trying to figure out if you need to call the school, schedule therapy, or just offer a snack. Honestly half the time my kid’s mood flips completely after eating something. Blood sugar is underrated in these conversations. Before you go into full parent detective mode, try juice and crackers first lol. I’m joking but also… not really joking.
I’d also suggest paying attention to patterns. Is it happening on specific days? Monday moods could be the weekend ending. Friday moods could be something that built up through the week. My wife started keeping a little mental note of when our son would go quiet and we noticed it was almost always on Wednesdays. Turns out he had PE on Wednesday and he was struggling with something in class and felt embarrassed. We never would have connected that without noticing the pattern. Kids rarely connect their own emotions to their triggers at that age.
This thread is giving me flashbacks to when I was 9 honestly. I used to go completely quiet and my parents would either ignore it or overcorrect by sitting me down for a big serious talk which made everything ten times worse. What I actually needed back then was for someone to just sit near me and not make it a whole thing. Watch TV together, cook together, whatever. Just presence without pressure. I think kids at that age want to know you’re there without feeling interrogated. The moment it becomes a formal “we need to talk” situation, they shut down even harder. Keep it casual and low pressure .
Okay real talk, I’ve been through this and I tried EVERYTHING. The car trick, the side by side activities, the gentle questions, the waiting. What finally worked for my daughter was writing. I left a small notebook on her desk one day with a note that said “you can write anything in here and you don’t have to show me.” Three weeks later she showed me the notebook herself. She had written about a girl at school who was making her feel left out every single day. She didn’t know how to say it out loud but writing it down helped her process it first. Not every kid communicates the same way. Some need to talk, some need to write, some need to draw. Find your kid’s outlet 
The notebook idea from TechRunner1 is something I’m going to try with my own kid, genuinely. But also DeltaXHexWave, I just want to say, the fact that you’re asking this question and thinking this carefully about how to handle it already tells me you’re doing something right. A lot of parents either dismiss the mood or overreact to it. You’re doing neither. You’re looking for the right approach. That matters more than you know. Kids feel safe with parents who think before they react. Keep that energy and your son will come around 
Something nobody has mentioned yet is that sometimes kids are picking up on stress in the home without even realizing it. If things have been a little tense between adults around the house, or schedules have been off, or even something like a pet being unwell, kids absorb all of that and act it out as general moodiness without knowing why. It’s not always a school thing. It can just be the atmosphere at home. Not accusing anyone of anything, just throwing it out there as a possible angle worth thinking about. Sometimes the answer is closer than we expect.
FrontNexus raises a fair point. Also worth checking if sleep is off. My son went through a two week moody phase and I was genuinely worried. Then I realized he had been staying up late because his tablet was in his room. Moved the tablet out, sleep improved, mood flipped within days. We sometimes look for complicated answers when the basics have slipped. Sleep, food, routine. When any one of those goes off track, kids feel it emotionally before they feel it physically. It’s boring advice but boring advice is boring because it works