My 3205 FA keeps popping off its Upper tension...

Question

I owned the machine for a year before I took it out of the box (I had other machines) and the first time I used it the upper tension assembly went flying off. I managed to pop it back on but every time I try to sew now it comes off and now I think I am missing parts. WHERE can I get this fixed. I stupidly bought it from some dealer that where demoing in K Mart.