If the hook assembly is removable, take it out.
It is vital for one to check this part and disengage it as it is done when threading the bobbin then turn the handwheel. Most old models of sewing machines if not all are designed with a knob inside the handwheel that may be locked, making it hard to turn the handwheel.
Test by turning the hand wheel.
The first you need to do is remove the bobbin and bobbin carrier. If you have a Singer sewing machine, these singer sewing machine repairs can usually be done at home.
When you attempt to turn the hand wheel, the locked up sewing machine hand wheel just wont turn or turns only under serious force. I have a singer prelude 8280 and I cannot move the hand wheel in either direction.