I just picked up my 2010 Mac Pro (tower) that had the same issue of not waking up after sleeping. Turned out it was a bad power supply! This issue had dogged me for so long, but over time I noticed that I was having to press the power button longer to get the mac to boot (gave up on waking it and just set it to never sleep). Apple store diagnosed the problem as a possible faulty power witch or power supply. $200 out of warranty power supply later and all is well. Wish I had done it sooner!