I had the exact same problem occur a couple of weeks ago. I check the power supply and the voltage appeared to be correct but not sure on the amps. I tried the old power supply from my Slingbox Classic to see what would happen since it is not the same power output. It actually allowed the PRO-HD to restart and operate properly. Since it was not the proper power supply and would cause it to over heat I didn't leave it plugged in long. This just confirmed that it appeared to be an issue with the power supply.
I ordered a new power supply for the PRO-HD from the Sling Media store last week. I figured 20 dollar is better than another 300 for a new unit. It arrived Monday afternoon and I plugged it in to the PRO-HD and the system is up and running properly. So it appears to be another case of a power supply issue.
I hope this helps you in correcting your problem.