How to unjam a stiff wheel on a Janome 6260?

I've no direct knowledge about your machine, but took a moment to search the fault code online and this is the manufacturer's advice provided, I hope it is useful:

Fault code "LO":
Replace the foot controller.

or...

1.Check for a tight or stuck clutch by the hand wheel.

2.The machine may be jammed. Remove the timing belt to locate whether the bind is on the upper shaft or lower shaft. If it is the upper shaft, there may be thread caught in the take-up lever or crank pin. If the lower shaft is binding, check hook clearance, hook backlash. Check the lower shaft rear bushing for any black ‘gunk’. If there is, replace the lower shaft and rear bushing.

3.Clean the sensor on the DC motor and check if the shield plate on the motor is not loose or hitting the sensor. If either the sensor or shield plate is damaged, replace the motor.

4.Check the uppershaft sensor. Make sure the shield plate is going perfectly thru the middle of the sensor. Replace the sensor and/or shield plate if damaged.

5.If you still get the error, replace the DC Motor or upper shaft sensor (in this order)

Rupert
February 2020