Water just stopped running into 2nd floor toilet. Water works in 1st floor toilet and in all the faucets, including the tub. We replaced the innards in that toilet just last summer. I checked the handle in the water line under the toilet, and turning it all the way either way doesn't do anything. I left it midway.

It's wicked cold out, but it seems that, with everything else working fine, a frozen pipe is unlikely. The pipes running into that toilet are not in an outside wall. Any ideas?

Shut the shutoff at the toilet to the OFF position and then disconnect the hose going to the toilet. Direct the hose into a towel and turn the water back on. If you have water, the problem is up in the fill valve (pop back and we can walk you through the repair for this). If no water flows, then the pipe is frozen somehow and you need to start trying to thaw the toilet pipe out. Here, you would leave the shutoff OPEN so water can trickle from the pipe as it thaws (just don't leave unattended) as you use a hair dryer to heat the toilet water pipe/shutoff.

Back to you...

Mark