Hi, I'm having an issue with my downstairs shower valve. We have not used the shower for a year or so but the other day ( I think the washer was running) the shower started running. I tried to shut off but nothing worked. I pulled off the handle and the covers to see what was going on. I can twist the knob around and an around but cannot get the shower to shut off. I have a Delta single handle monitor pressure balance valve, model 1300/1400. Any ideas, suggestions?