If your toilet has begun to run suddenly then the most common cause of a toilet running is due to a bad flapper.
When you flush your toilet the handle lifts up a rubber flapper to let water out of the tank and into the bowl.
When that flapper dries out it won't seal properly. Go to your hardware store and take the old flapper with you to get a replacement flapper.
Install the new flapper on the toilet and see if that fixes the problem.
If not then you either need a new float valve or the float valve needs adjustment.
Adjustment of the float valve will depend on what type of float valve your toilet has.