Why did I stop getting achievements halfway through Portal 2?

I've had this happen to me. I would guess your internet connection gave out or you were not logged in to steam. IF you're lucky the achievements were saved locally and will upload once you login to steam. But chances are you just need to get them again.

It can be caused by your Internet connection dropping out or someone else logging into your Steam account. Either way your session has been disconnected from Steam.

It is a good idea to have the console open because it tells you when achievements are not being earned if it's a different server.dll or you are disconnected.