Android Studio suddenly cannot resolve symbols

Question

Android Studio 0.4.2 was working fine and today I opened it and almost everything was red and the auto-completion had stopped working. I look at the imports and AS seems to

Answer 1

None of these methods helped me in Android Studio 0.5.8.

My solution was to delete ~/.AndroidStudioPreview directory (in Ubuntu). Sorry, I have no idea where is it in other OS. This directory stores temporary files and Android Studio settings, so I missed all my settings. But it works!