Sounds more like one of the wires' soldering came loose maybe? That should show with a physical examination.What is disturbing to me is that the loss of upper octaves was NOT gradual but rather "hear(sic!) yesterday, gone today"!
Is there something that can/should be checked to assure electronics have not been compromised?
Who is the best contact to inquire with about availability and cost of replacement mid/treble panels?
I am located in Central CT 06111