We recently had a problem where the head revision on the repository had the majority of the content mixed up. To restore the repository to a previous revision, you first update the repository (make a backup copy first) and then do the following:
svn merge -c -[REV]
Or
if you are using TortiseSVN like me, to go the repository -> Show Log -> right click on the revision -> Revert to this revision
Once done commit the code as a new revision and all should be fixed