There are a few things that could cause the tank to stay dry.
The water is shut off to the toilet.
The washer at the shutoff valve could have broken and clogged the valve or pipe.
The flushing mechanism inside the toilet could be broken.
If you're in a cold climate and the pipes are in an exterior wall( as previously posted) the supply pipe could be frozen.
Do you have a basement?
Where do you live?
Any other issues with water pressure?
Sometimes the supply tube gets clogged. Turn off the water to the toilet, remove the supply and see if you can blow through it. If you can, then try cracking the valve open just a touch to see if water will come out (keep a towel handy).
If water comes out of the valve, then the only other problem is the fill valve. You can try to fix it but you'll have to take it out anyway. Might as well put in a new fluidmaster 400A, and adjust as needed.