Try resetting the needle to make sure it's all the way up in the needle bar.
"[TABLE]
[TR]
[TD="width: 89, align: left"]248[/TD]
[TD="width: 73, align: left"]1980-81[/TD]
[TD="width: 65, align: left"]Domestic[/TD]
[TD="width: 548, align: left"]Zigzag; with buttonhole facility; drop in bobbin; white/green.[/TD]
[/TR]
[/TABLE]
"
Make sure the bobbin case, probably plastic is seated correctly on the hook assembly and if those two things don't do it remove the needle plate, slide plate, bobbin case and bobbin and verify that the hook timing is correct.
Or, take it back to the guy who touched it last and have him look it over.
Joe