The first questions I always ask is are you sure the needle is in correctly? Flat side to the left, thread it right to left. Are you sure you threaded the machine correctly? re thread it to be sure. Did you put the bobbin in the bobbin case correctly? The thread goes in bobbin coming off counter clockwise. Have you removed the needle plate recently? When you put it back the finger on the bobbin case base has to be in the slot on the underside of the needle plate. Is the bobbin case fully seated in the machine? And finally, are you holding on to your thread tails for the first few stitches?
Cari