DIAGNOSIS The control board has partial power, but the LCD screen or its connection is dead. This points to a failed display or a loose ribbon cable.
FIX Unplug the grill. Remove the control panel cover (usually held by screws under soft-touch buttons). Check the thin, flat ribbon cable connecting the display to the main board. Reseat it firmly at both ends. Plug the grill back in. If the display returns, the cable was loose.
IF THAT DIDN'T WORK The LCD screen itself is faulty. You must replace the entire display assembly. On the OG701, this is often integrated with the main control board, requiring a full board replacement.
HEADS UP A blank display with the fan on means the grill may still be receiving commands if buttons are pressed blindly. This is a safety issue. Unplug it until repaired.