We don't have an external SVN server. I was working on a PC with Windows 7 Enterprise, and I was using Eclipse subversion plugin as an SVN client. The problem in my case occurred when the commit of a file took to much time due to network problem that forced me to restart my PC.
After the restart I started getting this error: XXX file already locked.
The solution was to install (TortoiseSVN), right click on the SVN project folder --> TortoiseSVN --> cleanup.
I hope that this post would be a help to someone.