Water just stopped filling as though the outside main line was shut off, which it is not because the other toilet works and the sink and tub in the same bathroom as the non-working one both work.

First check the fill valve, be sure the float is traveling up and down freely. If it is then you have to cehck the supply line under th tank. There is ,should be, a valve under thae tank. Shut it off and disconnect the exit side of the valve, slightly open it, do you get water? If no then the valve itself could be the problem. Back to you for your results

Have you taken off the top to see if there is an obvious problem? You might need to replace or free up the inlet valve. A ball floats to shut off the water, if for some reason it breaks or doesn't drop the water will not fill. Because of cold weather a frozen line should also be checked. Does it run near an outside wall? Do you have a basement or a crawl space that gets cold?