water valve to toilet will not shut off and the water is running more and more even when off
This just happened to me yesterday I kid you not.  Not rushing water but a slow drip that accelerated overnight and went down thru the wall all the way to the basement.

The problem was that the rubber washer in the toilet fill valve had corroded because of water.  No problem when attached to the toilet but when I needed to it turn off it could not seal completely and then got worse over a few days.  .

Interested parties mandated that I call a plumber, and for $450 2 guys came and put a new toilet valve shutoff on the water supply line.  It is very nice. The other option would have been for me to bring the old toilet valve shutoff to the hardware store, match it with a new $15, bring that home and put it on.  

Benefit would have been savings of $335.  Risk would have been that I screw it up somehow and cause colossal house-destroying leak.  You do that once and people remember...