How do I fix my set-top box - it doesn't even switch on!

It did work ok as I tested it, I sold it on ebay, then upon receipt they said it didn't work and returned it for a refund. I am hoping they haven't removed/tampered with any of it as I am now £20 out of pocket plus the original cost - any help/ideas appreciated!! What is likely to have been removed if anything?

Michelle Trainor, July 2010