5D does not turn on ... except for the top LCD

After attempting to use it with a suspect battery running low (which has worked OK in the past), the camera now refuses to turn on with any battery. Strangely, the top LCD works, and when I turn the mode dial, it responds to that. However, the camera seems paralyzed below that and the main LCD does not work and even the data light (which flashes when you insert a memory card) does not work. Needless to say, none of the lenses focus, etc. etc.

At this point, I am wondering what would be the quickest repair option. I have a trip planned next weekend and I really would like my camera before then...

Check if the battery compartment is corrosion free, might be some aren't in contact with the battery

Have you tried pulling the small backup battery and letting the camera sit for 1/2 hour with both batteries removed. How old is the camera (is it the original backup battery .. maybe it needs replacing)?

I tried to remove the backup battery but I wasn't able to undo the small screw that gives me access.
I may have stripped the screw...