Hi, I'm having a problem with my bathroom tank. The water suddenly stop flowing to it after I flush. Can someone point me to where I can start looking to see what the problem is?

Start with the simple. Check the flap and see if it is able to move freely. Then check the float, and make sure it is not bent down, so that it will cut off sooner than it should. Both of those fixes are real cheap and easy to do.
I just thought of another thing. You haven't put one of those toilet fresheners things in there? I had one once that was clear plastic cup holding the freshener. When the freshener started to wear down, it of course got lighter and tried to go down the drain, causing the tank to not fill up, and the toilet to run. And to think I almost called a plumber until I reached in and touched the clear cup in the hole!