If you’ve verified that your water supply is turned on, carefully remove the lid of the toilet tank. You will probably find that the chain or strap to the flapper has broken or come loose. If the chain or strap has become loose this is an easy fix. If it is broken and you are a competent do-it-yourselfer you should have no problem fixing it. However, if you are not comfortable with this type of repair, contact a plumber.