The PRO-HD hasn't had an update in quite a while. Perhaps it's your video source that updated, and it came back up sending a 480i or 480p signal to the Slingbox. Check the menus in your set-top box to be sure that it's sending HD. Or, if there is someone at the Slingbox location, have them check what the TV is getting.
If that's not it, see whether the desktop player will show HD and whether setting up the video source again helps.
If no luck, post what the Statistics window shows for video width/height and frame rate, when attempting to force HD playback.