It's interesting that it was working fine and then quit totally. The antenna issue Neil mentioned was about an intermittent problem. It's worth a try but it seems different than what you're seeing.
After you rebooted the display did you repeat the procedure to "Add an Outdoor Device"? Maybe the display simply "forgot" the Atlas.
You have an Atlas and the HD display, do you also have an Access? If so is it still receiving a signal? That would show that the Atlas is transmitting. I think I'd try moving the monitor near the station (using an extension cord for power if necessary) and see it that helps. If not, my guess is that either the Atlas failed and isn't transmitting or the Display has failed and isn't receiving. Without a third device it's hard to tell which.