Turn off the valve then turn it back on.
If the tank fills one time then goes back to not filling and you have a Fluidmaster fill valve then replace the fill valve and supply line its shot.
Also another possibility is that you have a Watts Floodsafe supply line and it has nuisance tripped. Get rid of it and install a regular braided stainless steel supply line without the Floodsafe feature. Trust me you are lucky it didn't fail a different way...