I don't think there is a hard reset function on those units. And even if there was, you would need to get it to power on to do so, since hard resets usually involve either holding a button for a long period of time while it powers on, or a combination of buttons after it has powered on.
I have a 788 that I have had problems with powering down or the screen turning gray or black, but it ended up being a defective screen. When the screen got warm, it would do this. Since it was out of warranty by the time I figured out what it was (I originally thought it was a power issues), I now just run it with the screen set to about 80% brightness instead of 100%.
But I'm going to replace this unit soon with a used one that I picked up over the winter anyway. Oops.. I digressed