Problem with the lift chain
After you open the tank, you will see a chain that attaches the rubber flapper to the flushing handle. If this chain is too loose, it cannot lift the rubber flapper when the toilet handle is pulled thus the toilet won’t flush. To fix this, adjust the length of the chain so that there is enough pull to open the rubber flapper.
If you have checked all the above and can’t figure out the problem, it’s time to call a professionals!