I am missing my left pec muscle. Is it not there or is it just underdeveloped? What is the solution?

Losing weight should not have actually reduced the size of your pectoral or biceps muscles. Perhaps they they were not so visible when there was additional fat present. Strength is completely different and may well be a result of less developed muscles.  When a chest muscle is "not there" it is usually quite visible in any event.  This may be a functional, and not only an aesthetic question.  As with anything in medicine, the solution is to diagnose the issue first, and then come up with a treatment that fits the problem.

  from your description I gather your issue may be a congenital deformity called "Polands syndrome".  This condition presents asymmetry in the muscle with possible one sided weakness and aesthectically you may see asymmetry usually affecting one side the bodys upper extremities. The sternal head is absent but the clavicular head of the pectoralis major is present. I would recommend in your case either a pectoral implant or possible fat transfer.

If this in indeed a difference in the size of the pectoral and biceps muscle, the options would be fat grafting vs pectoral/biceps implants. It all depends on how much difference exists. A small difference would be treated by fat injections...if you have enough fat to harvest. But larger differences are better served by implants.