I am having the exact same issue.
I've purchased the PRO-HD in October and activated it on October 19. I has been working fine. Went on vacation, came back a few days ago and the box is not working. I have tried a number of different ports and cables on my switch that I know work, as AppleTV, Xbox and Playstation have no issues connecting to the network and to the internet. The PRO-HD, however is not connecting. The network LED is not on either, just as the poster before me mentioned.
I disconneded the device and let it sit without power for a few hours. Upon turning it back on, the center orb has lit up and continues to be in that state. Still no network LED, and neither Windows, OSX or web-based players are able to connect to it.
Has there been some automated patch installed that bricked the device?
I found this discussion:
Thanks to the answers there, I was able to narrow down the issue to my AC adapter. In my case, I owned an older, non-HD, version of PRO. When I upgraded to PRO-HD, I was lazy and did not connect the AC adapter that came with the new model, but instead plugged the Slingbox into the old adapter thinking it would make no difference. I am now powering the device with the correct adapter and my Slingbox is working once again.