I deleted a class and its corresponding test class. Shortly afterwards (~15 seconds but I wasn't counting) the IDE crashed.
I didn't catch any error when it crashed, but Windows did offer to relaunch the IDE
After the IDE had opened up again, I tried to open the project again but was met with this message in the Output console.
C:\Users\logan\Documents\The Welkin Suite\Projects\NZ Trade Group\NZ Trade Group\NZ Trade Group.sfproj : error : Object reference not set to an instance of an object.
The sfproj file still contained <compile> elements that referenced the deleted classes. I removed the two deleted files from here and was able to load the project again.