I have had this happen twice before. To fix it, I have had to unwire the enum from the case structure selector and wire a numeric constant instead. This will change all of the case structure enum values to numbers. I then check for a repeated number, which gives me an indication of where the problem is. After fixing the error, re-wire the enum and all of your case names will come back.
The last time this happened, after wiring the numeric to the case structure selector I immediately deleted the numeric and re-wired the enum back to the case selector. Then I checked the case values, and sure enough one of the enum definitions was repeated. This has happened so infrequently that it has been hard to pin down the exact sequence of events that causes this to occur, but it seems like the case structure occasional doesn't update when a change is made to the typedef'd enum.
Give it a shot and see if this helps you find your error.
