OK so it is not software as you never get off the bootPROM screen (the one in the TiVo BIOS) and therefore no hard disk activity. And it will not be the power supply as you would not get this far.

At this stage it could either be a dud turbonet OR a dud debug port (the name for the edge connector your network card connects to). Unfortunately the only way to test is to put the turbonet into another TiVo and see what happens.