MAC MINI 2006 not booting after firmware upgrade

Hi. Got a 2006 Core 2 Duo mini and decided to "entertain" myself by attempting to install Mountain Lion, for which first I had to upgrade the factory 1.1 Firmware to 2.1 using some forum-famous tool. Everything went according to this youtube video, except that after the tool finished the update "successfully" it was time to shut down, then power up by holding the power button until the LED was flashing rapidly, two chimes, etc. I heard the chimes, but the mini just stayed on black and will not boot no matter what I do. I've tried the PRAM reset by keyboard (a windows keyboard) by using the win key instead of cmd as I read somewhere, tried safe mode, holding v, s, and their mother too... I even opened the unplugged machine very carefully, took the PRAM battery out and pressed the PRAM reset button... all to no avail after reassembling. The only thing I have not tried is using an apple wired USB keyboard, but idk if that would be crucial.

So much for entertainment!

Any ideas/suggestions/solutions would be greatly appreciated!

Thanks!