You probably don't need to re-install R; just remove or rename the
file .RData (it is probably located in your home directory (or "My
Documents" on MSWin). Then R should start without problems.
As for recovering the workspace, I believ that is a lost cause (unless
you study the file format and use a binary editor to extract/repair
objects in the file -- and even then, if the file was compressed, you
may be out of luck).