Just be honest with him. Say "I used the camcorder a few times and it worked perfectly. I had it stored for awhile, and I just tried to use it and it wouldn't even turn on. Do you have any suggestions on what I can do? I'll offer to have it serviced, and if it's beyond the point of no return I'll replace it."
I have no idea how much it would cost to get repaired, but I would suggest going to dad-in-law before you do anything.
Good luck
