This happens because after updates Android Studio uses API version 23 by default.
The following worked for me:
Press Ctrl + Shift + Alt + S to get to the project structure page. Go to the properties tab and change 23.0.0 to 22.0.1 (or equivalent to what you were using earlier) in the build tool area and rebuild your project.
If that doesn't work, go to gradle:app and then
compile fileTree(dir: 'libs', include: ['*.jar'])
compile 'com.android.support:appcompat-v7:22.2.1'
Edit v7:23.0.0 to v7:22.2.1 as shown above and sync gradle. This will definitely work.