My Motif MS8 had a sticking B key. I turned it off and sprayed some contact cleaner on the sides of the key and that freed it up nicely. I waited 30 minutes and plugged it back in and now some keys around it…A flat thru D… make no sound. What did I do? How do I fix this?
You did do a bad thing - contact cleaner does not work for keyboards … they function entirely different. There are rubber contact strips underneath the keys that make contact on the key contact board below. Your contact cleaner has now either contaminated the key contact board and/or the rubber contacts or even caused the rubber contacts to stick the the key contact board hence they no longer can flex and do not make contact.
You have to take out the key frame, remove the key contact boards and the rubber strips and clean it all - suggest to use white methylated spirits (Do NOT use the coloured one it tends to be too aggressive and corrosive to rubber and plastic)
I fully concur with wernersaurus. And the sticking key issue is most likely due to the plastic key itself cracking at the pivot point at the very back of the key (very common with these keys). The solution is to replace that key with a new one:
Style 23U key, note B - Syntaur