DIAGNOSIS That's the 'Child Lock' indicator. It's enabled. The control panel is locked out.
FIX Hold the 'Heated Dry' and 'High Temp Wash' buttons simultaneously for 3 seconds. The 'C' should disappear. The lock toggles on/off with that button combo.
IF THAT DIDN'T WORK A power interruption can sometimes glitch the control. Unplug the dishwasher from its wall outlet for 5 full minutes. Plug it back in. The 'C' should clear.
HEADS UP If the 'C' returns by itself, the touchpad might be shorting. Check for moisture or liquid spills on the control panel.