Yesterday, during a thunderstorm the power went out while I was watching on my HD PVR. After the storm the box was not working - everything looks fine but the TV was not getting a signal. I tried our other HD box (was not on during the power failure) and it was doing the same thing.
We connected the box to the tv using the AV cables and that seems to work. We tried multiple HDMI cables, so I've ruled out an issue with the cable. Any ideas why this may have happened and why BOTH boxes would be affected??
It certainly is possible that a voltage spike damaged the boxes (or TVs). If you have time during a power failure, it's always a good idea to unplug electronics until after the power comes back to avoid voltage spikes when the power comes back. It's good to have a UPS on electronics like PVRs or computers. These cable boxes are never actually "off". They are always in standby. The only way they are totally off is if they are unplugged. (A small UPS can be purchased for around $50 on sale. You plug the PVR or computer into the "battery protected" side of the UPS and any high draw devices into the "surge protected" side of the UPS)
It's also possible that the box is now trying to send a signal to your TVs that the TVs cannot accept (say 1080P to a 768P TV). Here's a link to a post which allows you to change the output format of the box if that's the case:
It's also possible to do a factory reset of the boxes under "Settings" - scroll up to Troubleshooting. This will delete all your favourites, etc, along with any recordings on a PVR, so this is always a last resort.
I've had this exact problem due to a lightning strike close to the house. The stray voltage blew an HDMI port on the TV (connected to a PC - also dead). Was the TV plugged right into the wall or a wall power splitter?
I also made the mistake of plugging things into an extension cord without ground... bad as there is no grounding. I now plug into a power surge protector that is plugged in to an outlet away from the outside wall. Good news is that TV's these days have many HDMI ports, so one down isn't the end of the world.