Originally Posted by Rastabaxus
It's where the arm attaches. The pedal is ok...... As far as I can see, I have the arm, a nut to fasten it on and then a cover. Is there anything I'm missing?
Apparently it's the left crank arm that's falling off and the bike has a square taper nutted bottom bracket.
I expect the crank arm is ruined by now. It was probably installed without enough torque initially and, once it was ridden while loose, the crank hole was distorted to the point it will never remain tight. I think you are going to have to replace the crank arm and, this time, be sure it is installed correctly.