Thanks, friends! very useful info. I just needed to include the keyword 'hook' in the Google search. Though I've owned this machine since 1972, I've never had to learn about it, because it just goes! But suddenly, the needle is hitting something in the bobbin mechanism, so the machine cannot complete a stitch. I think something has slipped, the mechanism stops abruptly with the needle just entering the bobbin space. Will follow the advice you've offered, fingers crossed!