why does toilet shut off no longer turning off?

[abfab]

A few months ago the shut off was working perfectly fine when I had to replace the toilet flapper. Today I tried it and it won't shut off... not even close. I tried WD40 in case it was just stuck but there's no difference. Is there a reason it would suddenly stop working?

 

[havasu]

If you have a Fluidmaster float similar to the posted image, it is possible the valve is getting stuck before the black float rises to the top to shut off the water, and the excess water is spilling into your overflow tube.

If you have the conventional arm with the ball float, I would recommend replacing it with this Fluidmaster. They are easy to install and last for years.

 

[abfab]

My issue is with the water supply shut-off that is outside the toilet on the floor. sorry if my post wasn't clear.

 

[havasu]

I gotcha. Shut off valves will go bad after years of non-use, unless you take the time to exercise them occasionally. Most of us just leave it opened until something happens to cause us to shut the valve down, which is usually the time we realize the valve is frozen and also needs replacing. You can probably shut off your main water and tear apart this valve and fix what is no longer working, but it is usually easier just to scrap it and replace it with a new valve.

 

[Caduceus]

To add to what havasu said, the replacement should be a ball valve type shut off. At any home improvement store the cost difference is about $1 and is a more reliable valve.