I had this problem when a primer flowed back into the firing pin's hole in my Taurus 605's recoil shield. You, or a gunsmith, need to disassemble the gun's lockwork (be sure you know what you're doing when you remove the side plate, or you'll booger it up), take a punch of the proper diameter and tap the primer back forward and out of the hole. This will free up the gun. After that send it in for the "M stamp".

When this happened to my Taurus, it was the carry ammo that I'd had in the gun for the last six months. The first round tied up the gun, yet people think I'm crazy for carrying more than one revolver.