Water level in the tank is too low. Check the water valve, as it may be turned off. The water valve back on and see if it refills to the correct level and the toilet is flushing normally.
The rubber flapper may be bent or damaged. The rubber flapper may need to be changed.
The lift chain is not working. You may need to adjust the length of the chain so that the chain can provide pull sufficient to raise the flapper off of the flush tube and allow water to flow when the handle is pulled.
All these solutions can be don’t by yourself, but if you are unfamiliar with any of these contact your local registered plumber.