When MC launches, it creates a zero byte file called lock.jmd, that prevents two different instances of MC from modifying the library at the same time. Once locked by the first MC, any that come after will only have read access to the library. MC should delete that file when it exits, assuming that this is the root of your problem, we should try and establish why that did not happen.
You don't say which operating system you're using...
Assuming you use Windows XP:
Make sure MC is not running.
Open Windows Explorer and browse to: C:\Documents And Settings\[YOUR USER NAME]\Application Data\J River\Media Center 14\Library\
Look for a zero byte file named lock.jmd and delete it.
MC should now start normally.
If you're using Vista, you're looking for the same file, but the path will be different. Something along the lines of: C:\Users\[YOUR USER NAME]\AppData\Roaming\J River\Library\ (forgive me if that's not quite precise)
If the lock file exists, and you delete it, and MC loads normally, you should be sorted.
If the problem persists, you will need to post back with more details about MC version, operating system etc.
If you need to do that, you will find a "System Info" entry on the Help menu. Clicking on that will place the info on your clipboard so that you can easily paste it (Ctrl+V) into your post here.
Hope that helps,
-marko
