Bette,
First let me ask this, did it do this before it was serviced?
If yes, the service guy didn't fix it.
If no, the service guy caused it.
Either way, I'd start by taking the machine back to him.
Elaine says it's a tension problem. Probably the top. If the tension is not right on top it will double loop, tangle up, give you rats nests and so forth.
I have a Kenmore that's doing that. If I run the top tension way tight it sews, if I set it at a normal tension it won't go three inches.
Joe