Make sure you have threaded the machine from right to left, it sounds like the needle thread is trapped under the lower looper thread and cannot form a stitch.........start the threading over from scratch like this:
- Lower Looper
- Upper Looper
- Left Needle
- Right Needle
Is there a diagram for how to thread on the inside of the door ?
Follow that slowly and carefully to make sure you have not missed any thread guides on the way, as each one needs to be followed through to ensure correct tension and thread progression.
If you are certain that all threaded correctly, did you have a fabric jam and pulled it loose ? If so, the timing may well be affected.
Post an update and we'll get this right.