It could be two things.
1) Make sure the supply shutoff at the wall is open.
2) If that isn't it, it may need a new assembly cap, they sell for $1.99 at Lowes, and come with instructions on the back of the card.
A. Turn off water supply. Flush toilet. Remove refill tube and snap off cover (A)
B. Remove float by gently squeezing float arms inward. (B)
C. While holding valve with one hand, twist cap 1/8 turn counter-clockwise and remove. (C)
D. Lift out center tube. (D)
E. Use pliers to remove strainer from center tube. (E) Clean strainer and cap by rinsing under running water.
F. Re-assemble valve by reversing the above procedure. Note: If the main valve is worn out, (look inside cap), cleaning will not suffice, and a Replacement Cap Assembly, Hunter P/N R528 will be required.
Note: If the fill valve turns on and off by itself, or runs continuously without spilling into the overflow tube, the problem is not the valve. The fill valve is running due to a slow leak in the tank, probably caused by a flapper. Either worn out or misaligned.