I assume you've verified all the batteries are good?
I've taken that model apart before and fixed it for my neighbor, twice now. Removing that U clip will allow the gearhead to be pulled off. Tilt the black part down to avoid dropping gears on the table. Once the gearhead and battery clip are removed, you can split the red case carefully. It's electrically simple -- there's nothing in there but the motor, battery and switch contacts, which can be further disassembled. The problem is almost always bent or oxidized contacts.