Welcome from the central valley of CA.
Is your thread old, is it coming from wooden spools? That might be old stuff and weak.
Or, have you checked under and around the bobbin to see if it is gunked up with thread or lint? How about checking to see if you can thread it differently, or if it is in the right track from the spool to the needle.
And check that needle, have you changed it in the last few years? It might have small nicks in it, like a tiny crochet hook.
Hope you get it fixed soon and get back to sewing.