@staceb:
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:
http://communityforums.rogers.com/t5/TV/Nextbox-3-forced-resolution-to-1080p-issue/m-p/401480#M43723
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.