It is sometimes possible to open the .LDB file with IE or Notepad and in it, you would see the name of the person AND the name of the computer holding the lock. Then all you need to do is find that computer.
However, if it isn't you yourself holding open the lock, then you will have network-based locks. It is possible using Resource Monitor to look at file locks, one of which would be the .LDB file in question. In the RM Locks display, you would see the Process ID of whatever process is holding the lock. If you use the Task Manager and look at processes (to show processes for ALL USERS), you could then find the ID and kill that process. If you do, that should release the file.
HOWEVER, when examining the RM Locks display, verify whether that same process is ALSO holding a lock on the .MDB or .ACCDB file as well, because in that case you have to be a bit more careful. The ideal case is .LDB locked but nothing else locked, because if you kill the process, you are done. If the database file is ALSO locked, it is less safe since there is a chance that some actions are still pending.
If you can ID the computer, though, you can find that PC and manually exit from Access there. I've done that more than once.
The ultimate "unlock" is to reboot your system, but that risks DB corruption so that ought to be a last-resort type of action.