Portal 2 crash?

[Feral_Dogs]

So here's what's happening: I bought both Portal games, Portal works just fine- played it through already- and Portal 2 worked fine until last night. The menu works, the loading screens do their thing, and then when the last blue loading dot is done instead of going to the game, the screen goes black and something (not sure if it's my computer or the game) makes a loud, chugging "DRRN-DRRN-DRRN" noise until the computer shuts off.