Quick and dirty test is to try and back drive the leadscrew. Best if there is a nut on the end but if not suitable soft packing and parallel grip pliers or similar to get a hold on it should suffice.
If the screw turns relatively freely it will, in conjunction with similar observations of the gear train, help you narrow down where the drive has been lost.
Shear pins or similar weak links like Myfords Tufnol gears are normal in feed drive trains.
Sometimes designers get too clever by half.
Most annoying one I ever saw was a shallow plastic key between gear and shaft instead of aproper metal one. Pretty much unfindable wiithout the book or a full strip.
Clive