Do ALT+F7 first to verify you have no X running already. If not, then
This should list all files including the hidden ones (the ones that start with a "."). If it shows too many files and you can't view the top files, then hold down SHIFT and press your PAGE UP key to scroll (use SHIFT+PAGE DOWN to scroll down). If you see the file named ".X0-lock" then issue the following command:
rm -rf .X0-lock
This will not confirm that it has been deleted but it will be deleted. If you want to make sure that it's gone, you can do another "ls -a" or you can issue the option for rm that will ask you before you delete it (man rm
for more info). Let us know how it works.