When you powered back up, was the TV still connected?
If so, it's possible that your 5450 treated the TV as the primary display. In that case, you could turn on the TV, then use either Catalyst Control Center or Windows' "Display Properties" Control Panel applet to reconfigure the primary and secondary displays. Alternatively, you could disconnect the HDMI cable, shut down everything, and start it back up again. W7 should automatically detect the modified display configuration.
if not, then it sounds like your 5450 got "stuck" on your HDTV as the primary display. The easiest way to fix will be to reconnect the HDTV (or any other device you might have available with an HDMI connection), at which point you'll probably find that the primary display is through the HDMI connection. Then reconfigure displays as noted above, and carry on.