I was editing a mapping by disabling two reject rules on a target table when designer stopped responding. I killed designer and tried to restart it but it doesn't start on that particular workspace. It got freeze and the workspace got corrupted.

To fix this corrupt workspace, delete the following files :

WKSP\.metadata.plugins\org.eclipse.core.resources\*.snap

WKSP\.metadata.plugins\org.eclipse.core.resources\0.snap

When a crash occurs the eclipse framework may persist some information about the workspace in those files. It looks like they were corrupted which prevents the designer to start.