Sometimes repositories move, or they simple change a url and then you are left with a checked out version that no longer can do updated.
Subversion ships with a "switch" tool that accepts a "relocate" input.
The correct usage is:
svn switch - -relocate http://old.url.com/trunk http://new.url.com/trunk/
You have to be really careful and double check the url's or else you will be left with a broken repository.