Inspect four pads with green circles (Picture #1), they should be fully covered in solder.

Picture #2 shows the center pad missing some solder and not making a good connection.

Picture #3 shows repaired solder pad. Test toothbrush battery and motor operation without toothbrush head. If toothbrush is now working, there may be no need to replace the battery - if battery is providing between 1.2 and 1.5 volts and motor is actuated by button presses, stop here and reassemble the toothbrush.

If battery voltage is less than 1.2 volts, consider testing motor operation and button presses with two tests; using a AA battery and a pair of wire leads touch one end of leads to battery ends and the other wire ends to: A) left board pads (to test motor leads) and then B) to right board pads (to test battery leads, center pin is positive).

If motor doesn't start after Step 2 troubleshooting, continue to Step 3.