You didn't change [Media Type], you changed [Media Sub Type].
Unfortunately, Media Sub Type could have been set to either TV Show or Movie, depending on the source file. Did you reset only a few, or a whole bunch?
If only a few, you can change them back. You'll need to find them in a view in Media Center, then select them and open the Tag Action Window. If you don't have Media Sub Type listed as one of the available fields, click on the button in the upper-right side of the Tag Action Window, and choose Also Show > Media Sub Type. Now it will be. You can change them to the appropriate type from here.
If you changed a whole bunch, it might be easier to restore your Library from a backup. MC automatically makes backups in your My Documents folder by default (in the ~\Documents\JRiver\Media Center 18\Library Backups\ folder). They are clearly labeled by date. Check in that folder. If you have a library backup in there from recently, but before you made the mistake, you could just restore the backup, and it'll put you back to that point.
To restore the backup, in MC go to File > Library > Restore Library. Browse and find the backup you found, and proceed. You don't have to let it restore settings, as this isn't relevant for what you messed up, you only need the Library itself.