Peridically I have that issue with my 800. It happens when I remove the SDCard to my external card reader to update the card with EZScan. What I have found is that for some strange reason if I remove the SDCard from the radio (powered off) but leave the AC power applied, update the card and reinsert the card in the radio, the radio will not initialize. So now, I unplug AC power, remove one battery, update the card, reinsert it then put the battery back and AC power and no issue.
This week for the first time, I had to reformat the card to get it to work. Previously just removing power worked.
BTW you only have to remove power for a few seconds, not 5 minutes as suggested by another poster.