Press the Power Button on the front of the unit and hold the button for 5 to 10 second. When the unit resets it should download the guide.
I have been following the 721 problems on this board as well as a few others and I am starting to think that the 721 needs really consistant power.
I have mine plugged into a APC UPS system so the power is always good, this is why (and I am guessing) that I am not having the problems others are seeing.
I am guessing with the hot days there are fluctuations in the power level with everyone turning on their AC and other things to keep them cool, when the power drops the 721 freaks out.
Again it is just a guess, but my 721 is running great!