Never let someone leave...after they claim to have repaired anything
I don't know...the fact that you get no video signal has me worried.
Have you checked all connections?
I think you owe it to yourself...to give this person an opportunity to fix whatever he undid.
For what it's worth: The only way that the boot drive can change it's letter assigned (properly) is by doing a repair install of XP. Takes about 35 minutes, is painless.
Original problem: The only time I've seen a C: change to a D: drive...is when a repair install was attempted with 2 hard drives attached. XP saw the other drive and assumed it was C: and named the boot drive D:. I figured it out and undid it
The only other times that I know that a system will become flaky and do things like this: RAM situations and/or CMOS battery needs changing. Neither is earth-shaking, both irritating