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Sounds like the fill valve (the device that lets water into the tank and controls its level) is clogged. The best option is to just replace it. It's a pretty easy fix if you have some experience. A Fluidmaster 400 is a good option. It usually costs under $10 is widely available and comes with good instructions. You could also try flushing the fill valve out but this is often only a temporary fix.
If you had it replaced recently then the washer in the shut off valve under the tank has probably broken free and is clogging up the flow. I'm assuming the supply tube between the shut off and the fill valve is not kinked.