I'm with Candace on this one. I'd do just what you did and ask for help, then fix it myself before I'd go back into that shop.
There are a bunch of gals on this forum that know more about machines than those that messed with yours. If they can learn to fix them so can you.
Sew for a bit then slipped??????? Arrrgggggghhhhh ........
Might be worth taking sewing machine repair training and opening your own shop where you fix anything. Hmmmm, now that's an idea.
Joe