The correct answer was posted by @matt-helliwell if you're coming from a version older than 2016.2
.
File -> Invalidate Caches and Restart
If the above doesn't fix your problem, track this issue:
https://youtrack.jetbrains.com/issue/IDEA-157837
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…