This happened to me, and it was just a lot of dust that had built up behing the bobbin case, and there was also a huge amount of dust build up under the throat plate, So if you check that out, clean if out with a "Q" tip, place a a drop of oil on the bottom area where your bobbin case sits, (sewing repair man told me that all machines should get a drop of oil, every so often, sewing wears out this area.
I also take out the needle, and press the presser foot down as far as it will go, and run it backwards as well, do not use the same thread on the top as in the bobbin, I have found that not all threads like each other.
Put the needle back into its position, rethread your machine, hold both bottom and top threads while stitching the first 3 stitches, place material under pressure foot and attempt to sew - stitch 3 stitches then carry on stitching on your fabric.
If this doesn't work, take to your repair man, but it did work on my Bernina.
Best Wishes