When the C#/G# key is not closing anymore it could be caused by either a broken spring, or a spring that has jumped out of its position.
The spring is located underneath the C#/G# rod and sometimes hard to see. If it is missing, it is probably broken. Sometimes a spring is still visible, but has lost it's spring tension and therefore does not close the key properly anymore. In both cases, it would need to be replaced by a repairman. The broken shaft of the spring is most likely still inside the post and needs to be removed before a new spring could be installed.
Don't try to remove the spring shaft from the post if you don't have any experience or the proper tools. You could cause more damage. Instead try the following emergency solution.
For clarinet: If you feel comfortable taking out the screw of the ring key to unmount the E-ring key, take it off and then remove the rod of the C#/G# key as well. Wrap a rubber band around the pad cup of the key. (see picture) Then remount the C#/G# key again to the instrument and pull the rubber band around the instrument until it gets wrapped around the C#/G# post (see picture) and remount the E key. The rubber band needs to have the right length to be functional. If it is too short, it will be too hard to press. If it is too long, it will close the key completely.
For Buffet bass clarinet:
Since the G#/C# key mechanism includes 2 keys, the rubber band needs to be wrapped around the backside of the C#/G# lever.
A broken spring might be hard to detect.since it seams to be still in place
Wrap rubber band around the arm which presses down the C"/G# key.
Pull the band underneath the key backwards
Wrap the other side of the rubber band around the ring post