Thanks to everyone who took the time to post in this thread.
I have a Sony PRS-505 which I dropped. The power would not turn off however it worked fine. The soft reset did not work and I was pretty sure it was a mechanical problem with the switch.
Thanks to Igorsk's
instructions on how to disassemble the PRS-505 I was able to quite easily fix the problems. (If this looks scary, note that I only needed steps 1-4; only the top piece needs to be removed to fix this power problem.) The white plastic slider inside moves a metal pin--you need to make sure it is aligned when you put the top back on.
NOTE: you will need a 1mm (millimeter) screwdriver, which can be purchased in many dollar stores.
Good luck!