When the tank won't refill the normal problem is the refill valve is either clogged or the gasket has swolen shut. Often times debris goes up inside the cap of the refill tube and closes the opening so it will not fill the toilet. This often happens with Fluidmaster fill valves. Replacing the gasket on the fill valve will generally solve the problem. When the cap is removed, make sure you flush out the valve to remove any lodged material.
The other thing that can happen is the turn on valve will get stuck closed.