Has this happened all of a sudden or has it been progressive? ie: taking longer and longer to fill.
If it has been progressive then the tank valve (stalk looking thing) may not be the problem. If it happened suddenly then it is the culpret Im sure. These have a bad tendency to get build up on them and then the float mechanism gets stuck and wont allow water into the tank.
the first thing you should check is the 'angle stop' at the wall. Thats the little faucet that the supply hose attaches to. Make sure that it is turned on all the way.
If its is ok then go ahead and pick up a new tank valve at the hardware store. You will need a cresent wrench and a pair of pliers to change the unit out. There will be directions that come with the new valve. If you have a problem just ask it here in this thread. I will be here to answer you. (this afternoon that is. Have to do a repair job for my Mom this a.m.).
If this answer satisfies your question please click the accept button. I will not be paid if you dont.
You can always ask follow up questions after you accept. I will be here to answer them.
If for some reason you are not happy. Please tell me what I can do to put a smile on your face.
Thank you for using Just Answer. Please tell someone about us.
Best regards; The Home Smithy
After you have accepted it is important that you leave positive feedback.