Something similar has happened to my dad. He must have had a dozen shows recorded/saved, and then all of a sudden they were all gone yesterday. He called Dish Network and the tech lady gave him some lame explanations: 1) If the DVR detects an error then it erases all programming and 2) if you unplug the unit the same thing will happen. My dad's response? "I've been working with computers for the last two decades, and that's the stupidest damn thing I've ever heard."
Well, the DVR unit had been unplugged 4 or 5 times previously because it makes a loud hum, and it was keeping my dad up at night (he has the unit in his bedroom). Oddly enough it had been plugged in a full week when the programs were erased. He demanded that his DVR unit be replaced or else he'd send back every last bit of his Dish Network equipment, so I guess they will be sending the replacement.