Sounds like the inlet valve washer has adhered to the seating so stoping water inlet.
Depends on type but plastic ball valves have a large diaprham washer and it's probably stuck to the seating.
Turn off the water to cistern remove the float arm undo the large nut on the valve assembly and see if the washer has stuck. Pull it off get a replacement re-assemble.
Rob
July 2008