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

In my multi-module project, the problem was that version of "com.android.support:appcompat-v7" in module A was "22.0.0", but in B - "22.2.0".

Solution: make sure
1. version of common libraries is same among modules.
2. each of modules compiles without any errors (try to build each of them from CLI).