If you are sure that after putting in labels the head knob has been completely turned clockwise, thus locking the head in operating position, then most probably your head sensor is defective.
In this model it is, if I remember correctly, a small microswitch located to the left of the head assembly; a steel slider attached to the assembly moves down with the head, pressing a small metal "tongue" on the microswitch, which in turn presses a tiny prong sticking out of the microswitch. It looks similar to this:
The tongue sometimes falls off due to wear and tear. If this is the case, a temporary solution is to insert a toothpick or something similar to press this tiny prong on the microswitch, which will let you print, and order a serviceperson to replace this sensor.