I pressed something, and my object went curved I was modeling an object, using the Maya keyboard layout, and as I was pressing Alt+2 to move the camera, my finger slipped, and the object went... strange. I've observed this effect before, but I can't remember what it's called or how to (un)do it. The shape deformed, rounded off, and really annoying. What did I do, and how can I fix this?

Before:


After:
You most likely added a sub-surface modifier on your object. You can do this by pressing 

Ctrl+1 throughCtrl+5, adding a sub-division surface with the respective amount of divisions.

Because you were pressing Alt+2 before it happened, your finger probably landed on Ctrl when it slipped.

In your second screenshot, you do not have your object selected, so the modifier would not display. in the modifiers panel

You can remove it by going to the modifiers panel and deleting the modifier.