I had this same issue on my upright.
I found it was a flange on the Jack, that is conneted to the Wippen. This flange had slipped out of its housing. I had to take the action out and put it back in. See photo. The part that slipped out is circled in red. Make sure the jack spring goes back into its holes top and bottom OK.
You sould be able to see if it is that part that has slipped out when you take the upper panel and fall board off, then press the key.
It maybe something else, but this was my issue.
I had never done it before, but I managed to get it right first time. It's a bit tricky, but can be done. If you are unsure, get a technician.
Best,
Rob