I'm going to assume you've already made sure the water supply valve under the tank is turned on.
Take the lid off the tank.
Is the float all the way at the top and stuck? Try pushing it down. If it drops and begins filling, you may have one of the anti-leak designs of fill valve. If so, that suggests your flapper is leaking. Flappers are cheap and take just a few seconds to replace.
If the float is already bottomed but water isn't coming in - or you can juuuuust barely hear it, then the valve itself is the problem.
Original equipment valves are pretty cheap and generally not repairable (depending on the manufacturer), but Fluidmaster valves are everywhere, inexpensive and do their valving action with a rubber diaphragm. When that diaphragm fails, it gives little warning and suddenly the tank takes a long time to fill, if it fills at all. In my house the diaphragm is good for about a year or so. It lives under the black cap at the top of the fill valve - push down and turn, like a pill bottle - and usually comes up with the cap. Pry it out, new one goes in the way the old one came out, cap goes back on. Job's done. New diaphragms are about $2.50 - 3.00.
If it turns out yours isn't a Fluidmaster valve but the valve is the problem, replace it with a Fluidmaster. They're what I use in the few non-commercial toilets in the buildings I maintain.
Good luck with it.