I don't think it's POSTing at all. All that the fans and drives spinning proves is that the fans and drives are spinning. In fact what you describe is typical of a motherboard failure which, given what happened, is a distinct possibility here

Check that everything is firmly seated, cables, cards, RAM, etc., especially the graphics card. If that doesn't help, try clearing the CMOS in case it got scrambled. There should be a jumper on the motherboard to do that, if there isn't or you can't find it then pulling the motherboard battery out for a few minutes will achieve the same result.