Sounds like a fill-valve problem. May be calcium deposits blocking the flow. I would recommend replacing the valve. I use Fluidmaster 400A valves. Less than $10 at most Home Centers and hardware stores. While you have the valve disconnected, hold a rag over the top of the supply tube and turn the supply valve on and off 2 or 3 times. This should clear any blockage in the valve or supply tube. If you have the "ball type" fill valve, the Fluidmaster will eliminate it. Keep the (extremely simple) installation instruction since they also contain trouble shooting info for the future.
if you have never made a mistake, you probably haven't done much.