I know how frustrating this can be.
I don't know your machine, but here's some thoughts ...
Are you doing regular straight stitching? or something else?
If not, switch back to regular straight stitching ... and prove that it is working fine in that mode.
If it is, then gradually try other modes.
Did you re-thread both top and bottom thread?
Changed both threads to a totally different brand of thread? matching both to the same brand/type/weight?
Did you check your manual, for trouble shooting?
Checked your manual that you are threading both correctly?
Called your dealer for suggestions?