The .svn\pristine folder stores copies of each file as it was when checked out from the repository (a "pristine" version), allowing some operations to be performed offline and also allowing Subversion to send only changes to the file when committing.
I would guess that something (antivirus program, filesystem faults, user error?) has caused a file to disappear from the pristine folder.
A search in the Subversion archives gives a few pointers, however these are quite technical:
A more heavy handed approach is to remove the .svn folder and make a new checkout on top of the previous working copy (make a backup first of course!) - or discarding the old working copy and checking out a completely new one (copying over any changes manually).
Kind regards,
Daniel