System was shut down by me. Someone else took it to a shop opened it up and blew it out, closed up the case and returned it to me in this condition. I've gone over all the cabling thinking something had come loose and was reattached in the wrong location but everything seems correct. Any ideas?
The fact that you see the Standby Voltage indicator does not mean that the PSU is capable of delivering full power. Still, it's rare for something to fail just because of a cleaning.
I suggest that you try a few things. First, look at the front panel header to make sure that everything is still plugged in properly. The power button is pins 6 and 8. Actually, going back to basics, unplug everything from that header, connect A/C power and then touch pins 6 and 8 simultaneously with a small screwdriver. This should cause the system to power on. If that works, you know that the front panel support is what has been screwed up.
If you reach the point where you thing everything is plugged in properly yet it is still not powering on, try clearing CMOS. Unplug the system from A/C power and then remove the CR2032 battery from the board for 15 minutes. If it then powers up, you will need to go into BIOS Setup and set the date and time again.
Hope this helps,