Hi,
We shut off our toilet supply valve (it's been running), but something seems to have happened when I tried to turn it back on. I turned it counter-clockwise, and immediately heard the water start running again, but then (for some stupid reason) kept turning it. There was an audible *click* and then the water sound stopped. The valve no longer seems to work, or at least the toilet won't fill up. Thoughts?
Thanks for your help

In some of the older valves, you could screw off the washer and it blocks the hole or a piece of debris is blocking the ballcock. Shut off the valve (angle stop) , disconnect the line at the toilet -water will not pour out- put the line down into a small bucket and slowly open the stop. If you get no water, replace the stop. If you get water, you have a plugged ballcock. Replace it or disassemble and clean.

Don't know hat happens but really not worth trying to figure out. Stop by local hardware or big box store and pick up a new one. Only a few dollars.
Hopefully you have a compression fit type. Just get the correct size. Most common is a 1/2" inlet by 3/8" outlet.

Turn off water to house. Loosen nut around supply tubing, then loosen nut on input pipe and remove. Replace with new valve.

If you have the chrome, hard, type supply tubing, also pick up a new braided flexible supply. Much easier to work with and you won't have any problems with getting a seal around it. Check for needed length before going shopping.

If I had to guess, I would say that you have a piece of solder in the pipe that move to block the input port of the stop valve. The click you heard was it suddenly blocking the valve stopping the water flow.

Thanks for the responses, I'll investigage further tonight and see if I can figure it out.