Cause and Solution 1:

The upper thread broke, the thread came out of the thread tension disc or a thread guide, or the machine is not threaded correctly.
Check that the thread passes under the upper and middle thread guides, and correctly thread the upper thread.

 For details, see "How do I thread the machine? (Threading the upper thread)".

If the thread is not broken, clean the thread guides and the tension unit.

For details, see "Maintenance (Cleaning the thread paths of the upper threads)".

Cause and Solution 2:

The bobbin thread broke or ran out.
Check the condition of bobbin thread.

If the bobbin thread runs out, replace the bobbin thread.
If the bobbin thread breaks, reinstall the bobbin and bobbin case correctly.
If the bobbin thread is tangled in the hook, remove the thread jammed and clean the hook.
If the bobbin is scratched or damaged, replace the bobbin with a new one.
If the bobbin case is damaged, replace the bobbin case with a new one.