Let’s say for example you accidentally ran “svn update” and your working copy got the updates that it shouldn’t have — unfinished patches, etc.
What you should do is check the logs using:
svn log | more
Look for the revision number then:
svn up -r 1234
*where 1234 is the previous revision number you want to go back to.
Another reason why you should properly put comments when commiting to your repository.