Hmm... seems like a serious glitch in LN
I haven't heard or seen anything like this before !!
Could be a memory problem or somehow the interaction with the piano is somehow not properly 'completed'.
Maybe you can try your Sim (which is asleep in bed), to play piano again. If the game accepts, please let him play till he decides to do something else on his own. After that try to play the piano with another Sim and see if it works properly again.
If the game doesn't accept and the cheat "moveobjectson" doesn't work either, I really haven't a clue.
Maybe PK has any idea or heard anything about this ??
Otherwise it maybe an option to contact EA about this, but I doubt if they can help
Or go back to the last game save which didn't have the problem.... (if you have one)
Probably not the things you want to hear. I hope somehow the first option works !!
Good luck !!