First, make a backup copy of your project in case something goes more wrong.
Then, have you tried going to Play Mode and then choosing from the main menu Play > Apply Default Playback Template?
What a gem of an instruction… No. I had not tried that…
YES… it worked… I am so relieved and grateful!
I will remember that tip if it happens next time… THANK YOU for sparing any further hours trying to problem solve that one… Again, I would have never had arrived at that myself…