Leave it unplugged for more than a minute, then plug it back in. After the reboot, if you're still at a black screen, press the sysinfo button on the front of the 921. If the sysinfo screen doesn't come up, then press it again. It may take many seconds before it appears.
When it comes up, if the screen shows errors with your smartcard, you've got a problem. If it comes up normally, then exit the screen, and power cycle with the remote. If still a black screen, then press the HD/SD button once or twice.
If none of that works, your 921 may be dead.
And of course, check your video connections to make sure they haven't come unplugged.