Problems with a few keys (61)

[GrimmTeather]

I've had this problem a couple of times. 4 keys completely stop working. The first time it happened, they gradually started working again, one at a time.
C7, D7, D#7 and E7 are the keys that do it, but the C#7 is and always has been fine. There is no sound and they do not map in the sequencer.
Has anyone else had this problem? Does anyone know what's going on?

Alright, E7 has just started working.