can't get a machine to boot ...
it's a simple enough fault to diagnose for anyone with a little knowledge
for someone who is supposedly in the game - it's inexcusable not to be able to say why (fixing may cost more than the machine is worth ... but that's another issue)
the only thing in the guy's favour is that he's not lied to you
a less honest idiot would have told you a string of lies
if you have the machine back it's obviously easier to play hardball
I'd seriously consider seeking compensation - the small claims court is easy enough to get round
if you'd rather not
just write to the guy stating that in view of the circumstances you will be either
seeking compensation
or
refuse to pay given that he has failed to repair or give a solution.