Sometimes you can repair this by closing all editors and all perspectives, exiting, restarting the machine, and reopening WSAD.
SOMETIMES. Repairing the metadata is complicated, but may be worth it. Make a backup copy of your .metadata folder before you start mucking with it. Your first option is to try a fresh folder generated via a -data flag on a command line, then copy the .plugins folder over the new versions to save your preferences. Beyond that, try replacing files 1 by 1.
YMMV.