Here are some things that can fix this (in increasing order of difficulty):
Clean the project (Product > Clean)
Completely close Xcode and restart your project.
Delete the Derived Data folder (Window > Projects > click Delete button by Derived Data.) (Update: In Xcode 8 this location no longer appears. Supposedly Xcode cleans up Derived Data by itself. If you want to delete it manually you can find the folder yourself by going to Preferences > Locations and clicking the gray arrow by the Derived Data folder. Then delete your project folder.)
Click delete on the class, remove reference (not Move to Trash), and add it back again. (see this answer)