It is necessary to find out which of these is applicable. You can search for processes called ‘svnsync’ using the ps command:
If svnsync is running and you believe it has hung then you will need to kill it before proceeding.
If svnsync has died or been killed, and the destination repository remains locked, you can unlock it with the command:
svn pd svn:sync-lock --revprop -r 0 file:///var/lib/svn/foo
Do not remove the lock unless you are confident that there is no instance of svnsync running. If two svnsync processes are allowed to write to the destination repository concurrently then it is possible they could leave it in an unusable state.
(In particular, do not assume that a process will necessarily die following a kill command. You should check using ps, and if necessary issue a kill -9.)