Just reinstall.
It won't wipe your settings. If it does, there should be a few Library Backups here
<My Documents>\JRiver\Media Center 18\Library Backups\
The automatic Library Backups include all settings and the full library.
If you have trouble with MC launching at all (you never get the chance to pick from the built-in troubleshooter), basic troubleshooting steps would be (testing after each step):
0. Reboot. Sometimes something is just "stuck" (usually with the Explorer shell extensions, in my experience).
1. Reinstall the latest build "over top" of your existing install. This preserves ALL settings and won't hurt anything.
2. Uninstall the current build, making sure to DISABLE the options to remove the Library(!) and Settings. Then, reboot, then reinstall the current build (again, this will preserve all settings and the Library).
3. Uninstall the current build, DISABLING the option to remove the Library, but enabling the option to remove the settings (this will reset corrupted settings). Reboot, reinstall the current build.
4. If none of the above work (or if you're sure you have a good backup and you just want to skip jumping through the hoops, uninstall and enable both options (remove Library and Settings), and then reboot, and reinstall.
(You may need to re-license MC after some of these steps, but just ignore it and run in trial mode until you get it fixed.)
This last method will give you a blank install of MC, which
will then start up (if not, you have more serious Windows/Hardware trouble). Then, you can try restoring from the backups available (automatically in the directory I specified above, but you can move them if you want), which will get you back to the way you were before.
To restore a Library Backup once you're up and running, check the info in this thread:
http://yabb.jriver.com/interact/index.php?topic=76170.msg530471#msg530471