I can't afford a new fridge right now, and I probably won't be able to for a while. But the fact that the freezer is still working gives me some hope that whatever went wrong is simple enough to fix on my own. Are there any repositories on the internet where I might find a guide or a checklist on diagnosing the problem?

EDIT: Thanks so much for all of the help. Unplugging the it and letting it defrost for a couple of hours actually did the trick. It's now getting cool again.