Need some ideas here as to what might have gone wrong with M&P 340 today. I put one cylinder down range and then loaded another cylinder. One shot fired and then the next shot I thought misfired on me. It wasn't a misfire. I thought the infernal lock might have set on me, but it was unlocked. The cylinder is froze for some reason and the trigger/hammer won't come back.

I had this happen once on a Ruger LCR and it was the return spring that snapped on me. But when I pulled the sideplate off the M&P, everything looks right.

Any ideas as to what I could look into to find the problem?

Thanks!