Double-check to make sure your needle is in correctly, that it's properly threaded, and that your bobbin case is properly inserted. Any one of these things can be the cause. Check the screw that holds your presser foot on the machine -- I've had Singers on which that screw loosens and it will mess up the stitching. I know there are many others who have far more experience that me, but those are some primary things.
Jeanette