Replace Black Water Valve with full holding tank

I don't know if I've ever seen a valve 'break'

I could believe that the cable came off of the valve is stuck but I bet that with some assistance you'll be able to get the valve to open)mind you it may leak a little liquid...

Get down there open up the under-belly and see if it's a loose cable of out of adjustment-you'll probably need a small allen-key if it's the cable...