Handwheel is stuck? No bobbin jam

Have you removed the bobbin case itself to see if it has any thread or compacted fluff there? Also sometimes we forget to return the thread lever back after filling a bobbin. this lever blocks the handwheel from turning. If it is a metal bobbin case it may need just one or two drops of oil.

Failing these things, you can turn the machine upside down and remove the bottom plate to see if any of the screws that move the handwheel mechanism have tightened over the years. Otherwise, I guess it's a repair job. Hope this helps. :)