Unthread the machine and turn it back so you can take off the bottom cover.
On the left side, under the needle area, you will see two gears at right angles to each other. The gear that's oriented vertically (when the machine is standing up) might have come loose and is not engaging the horizontal gear. There should be a small amount of play between the two gears - they should not fit tightly together. When you turn the handwheel, the vertical gear drives the horizontal gear.
If the gears are too far apart, turn the handwheel and look for an allen nut on the bar that turns the vertical gear. Loosen it with an allen wrench, slide the bar to the left to engage the horizontal gear (but not tight) then tighten the allen nut. When you turn the handwheel, the gears should now be engaged and the bobbin case should spin.
If the teeth on those gears are broken off, they're unfortunately going to continue to crumble and replacing them is expensive - you can do it yourself if you get the parts and save a lot of money.