Hey my right gripshift won't turn, need help
Sounds like the gripshift itself is broken.
The usual process to diagnose jammed shifter or derailleur problems is to disconnect the cable at the derailleur, check the derailleur will move through it's full range, then tension the cable and work the shifter to make sure it pulls in and pays out cable.
If the cable is loose but the shifter will not move something must be jammed in the shifter itself. Try disconnecting the cable at the derailleur and pulling it out of the shifter. The head of the cable might have come off and has gotten stuck in the shifter.
If you can't get the shifter to move with the cable pulled out replacing it is probably the only option.