I live in a one story, two-bath house. One toilet was used by my aunt, now deceased, and the other is the one I use. Today, my toilet didn't flush. I looked in the tank and there's no water there. I was able to pour water into the bowl to flush it. Then I poured water into the tank to see if it would flush properly, and it did. I turned the shutoff valve (I think that's what it's called) to see if water would flow into the tank. Nothing happened. I can't imagine water not being available to the toilet all of a sudden. The other toilet is fine, so I guess this problem is isolated to my toilet.

The toilet tank doesn't have a float-ball; instead, it has a tall device with a thing that supposedly travels up and down (?). I would really hate to think that the water is suddenly flowing somewhere, like a broken pipe. I would think if that were the case, I'd hear something at the toilet or the water tank. (?)

Could be a frozen wall pipe, not uncommon with these temps.

I hope that's all it is. I'm pretty worried—the kitchen sink is on an exterior wall and it's fine. My bathroom is on an interior wall and the house is on a concrete slab. A burst pipe would be a disaster.

Does sink and tub (if there is tub) in that bathroom have water?

Raise and lower the tall thing (float) and see if the water comes on. It may just be just the fill valve.

To find out if you have water to the toilet, simply turn off the water supply valve below the toilet, the whole float valve assembly can be removed by a retainer ring below the float, just slide it down and pull the whole assembly up, and crack the supply valve open. If you have water you know it's the fill valve, which may be just clogged, and can be flushed out/ or replaced for little cost at home depot. I just replace the float valve, and use the original stem you already have.

How to Repair a Clogged Toilet Float Valve | Home Guides | SF Gate

And

https://www.youtube.com/watch?v=WYWIOiGWFYY

Once you determine if you have water, or NOT you can proceed from there. The above process takes all of 10/15 minutes.