Machine is blocked
If you can't turn the flywheel , then most of the time the bobbinwinder is in the winding position.
Push the bobbinwinder to the left
Sometimes you will have to move the bobbinwinder a few times from left to right, end with the bobbinwinder to the left
Is it not the bobbinwinder ?
Then the hook is dirty with dust and bits of thread.
Use the occasion to thoroughly clean the machine: the hook and race area and the needle bar.
Take off the upper hood on the left side of the machine, remove all dust! Use a bit of WD40 on the needlebar on the spot the needlebar crosses a kind of bloc. Let it soak for a moment. Try to get the machine moving by turning the handwheel.you can use a bit of force!
If it works, let the machine turn for a little bit before threading and sewing