switching the tower on/off many, many times; and uplugging and replugging in the power cord
That's a great way to cause all kinds of problems. It's often a good idea to power down everything and reboot, even if you have to force shutdown, but this is the sort of thing you do once. Either it helps or not.
My guess is that, after so many violent power cycles, you accidentally jiggled the monitor cable, which made it work again. Unfortunately, you now have a bunch of corrupted files...and there are probably others you don't know about that will cause problems down the road.
You should probably just bite the bullet and wipe your drive and reinstall everything. Sorry.
Thanks for taking the time to read and respond.You're right about everything--except for the monitor cable jiggling causing the monitor to work. Why it randomly didn't work for days at a time is still a mystery; though I read that many folks who bought the HD Cinema Display around 2008, like I did, had the same problem. Several of those folks said it was the power brick gone bad.
The mac repair shop said the same thing you did: i probably had corrupted directory files caused by all those on/offs. I did end up wiping the drive and reinstalling; and it fixed my issue (not being able to render disk flipbooks)...