If the water valve was off, turn it on and monitor the water level in the tank to ensure that it refills to the correct level, then flush the toilet to make sure it's flushing properly again. Remove the toilet tank lid and inspect the rubber flapper.
The most common reasons for a valve to slow down or no longer fill the tank. Fluidmaster recommends flushing debris out of the valve / water system first in .
Flush the toilet after these adjustments and see if the water fills properly.. If this is the root of your issue with your toilet tank not filling properly then it may not be .
Is your toilet's tank not filling with water? These helpful tips from Roto-Rooter will fix that problem.