I just reset my Diplomat ADP3300 by holding down the four leftmost buttons for a second. Then I used the plus and minus buttons to reset the time on the clock. After this, the oven and grill worked fine.
I assume that this is a safety feature, so that if you left your oven or grill on, and there was a power-cut to your house, then if hours later, the power is restored, the grill won't come back on unattended and maybe start a fire.
Giles, March 2010