Resharper stops identifying keywords and libraries.

I'm using Ultimate. Sometimes, ReSharper will stop recognizing keywords like void and string, as well as libraries like System. Neither clearing the cache nor rebooting the computer seem to help. In a solution with several projects, some can work and others may not. If I've had it working normally in one project, it continues to work normally in that project. However, once it stops recognizing anything, it continues.

Anyone know how to reset it for these projects?