Well if it was running and you left it on then came back to this mess its very likely that something fried. Does it smell like burnt electronics? Open up the case and look over the motherboard for any discoloration. From there its going to be a process of elimination on troubleshooting.
Do you get any beeps or anything? Have you opened the case to make sure all your fans are still spinning? When you power it on do you hear the hard drive making any noise?