Try deleting "Lightroom 5 Catalog.lrcat.lock" file (look in folder with catalog). Make sure the extension is 'lock' before deleting.
Also, via Task Manager, make sure you have no other instances of Lightroom running (look for lightroom.exe on processes tab).
I deleted the lock file as I had nothing opened in the task mgr. Thanks Rob. You saved the day for me! I really appreciate it.