My [23/F] boyfriend [21/M] of 3 years ignored me when I came home from work today. We had an argument about it, that made him cry, and he left me alone for the rest of the day.

I work full time and my boyfriend goes to university, when I come home he's always excited to see me, and he gives me a hug or kiss, etc. That didn't happen when I came home today. He was on his computer and didn't say anything to me so I said, "Hey honey I'm home."
My boyfriend ignored me and pretended I wasn't there. I said his name and he replied back in an annoyed tone. I asked him if something happened today and he told me to go away.
I tried to get him to talk about his day until he said, "Can you go away? I don't want to talk to you." I was really upset at the way he said it, and we got into an argument about how he was ignoring me. It left both of us feeling sad, but my boyfriend cried at the end of it and told me to leave him alone for the rest of the day.
I respected his wish, and he followed through with it. My boyfriend didn't eat dinner with me today, and decided to sleep on the couch.
It's 1:45 am and I basically haven't interacted with my boyfriend for the past 24 hours. I miss him so much, but I don't want to upset him again. I don't know why he's acting this way. This behaviour is extremely abnormal for him. He's never been this upset over anything before, or refused to communicate with me.
I think something happened to him at school that he's afraid to talk to me about, but it must be very sensitive if he refuses to discuss it with his partner of 3 years. I don't know how to bring it up without upsetting him. Maybe I could write him a letter? Or a text message? I guess in any way that makes him feel more comfortable.
tl;dr: I came home from work and said hello to my boyfriend. He didn't acknowledge me or reply back. I asked him if something happened and my boyfriend said that he didn't want to talk to me. We got into an argument, and he cried, then left me alone for the rest of the day.


Source.