I got drunk
And my girlfriend won't talk to me

Maybe that's the reason? Maybe you did something stupid when you were drunk? We really can't give you a worthwhile answer based on what you wrote. None of us were there so we need more details.

Sounds like there is a reason, not one you accept as a problem perhaps.

Sounds like you got drunk, acted like a drunken fool, and that's why she's mad. Of course you didn't give a lot of info to go on, but based on the very limited info you gave, I'd say it's not too hard to figure out. You did something stupid or annoying while you were drunk, which people often do when they're drunk, and now she's mad.

Was anyone else around when you got drunk? If so, they might be the people to ask if you can't remember doing something to upset her.

May I ask how old you are and if you should have been drinking in the first place?

Was she with you when you got drunk? Did she have to take care of you after you got drunk?

How often do you get drunk? Has she gotten upset about your drinking before?

Not enough information.

Anyway, she could be mad at you because:

1. You were drunk
2. You did/said something while drunk that she didn't like
3. She's already had some issues with you which you didn't perceive as a problem, and your getting drunk might have been the last straw.

What you might think as normal, casual, not-to-be-taken seriously, may be something that she took to heart.

First apologize to her, and then ask her where you went wrong...