Sensor may be falsely indicating warning... or there may be a jam
Likely to be a thread jam...?
• Check around and under bobbin case
Check under Needleplate for feed teeth lint trapped in there
• Takeup lever on top shaft (VERY difficult to see at times, as the thread can jam between the shaft and armature… good light and a magnifier is recommended) (The take-up lever is located directly above the presser foot of a sewing machine. It's the part of the machine that pulls the thread from the spool to feed it through the machine and lifts the thread back up out of the cloth after a stitch has been made. Simply put, it's the part that goes “up and down” as you sew.)
• Handwheel near bobbin winder… (inside the body, on the spindle, even the Handwheel at times)
What can you see ?
If the Handwheel is stiff or locked up, there is a 95% chance of a jam in one of the 3 places mentioned
Takeup crank can be so bad as to have jammed BETWEEN Shaft and Connecting rod... it needs to be dismantled to resolve, and is not for the untrained unfortunately
The other 5% is where sitting idle for a long period causes issues of corrosion or old oil forming ‘glue’