Hi,
Is it humming but not spinning when you try to turn it on?
If it is humming, check if you can you gently turn the spindle by hand to see if it moves or even if it starts to turn by itself (if the power is switched on) once you try to turn it.
''If so'' it may be that there is not enough power to initially start the motor rotating or the spindle is jammed somehow.
''If not'' you may have to find out why the motor is not getting any power.
Couldn’t find a guide for your model but here’s a [https://www.youtube.com/watch?v=9mCaSk08Bp0|video] that shows how to replace the batteries in a Genius 3765. Hopefully this is close enough to your model to be of some help. At least enough to open the toothbrush so that the motor action can be checked.