When I push the toilet flush handle, nothing happens. Why?
This is because the linkage between the flush handle (trip lever) and the flush valve has become disconnected somewhere inside the tank. To fix it lift the tank lid and inspect the trip lever and all the connections made to it going down to the flapper at the bottom of the tank. Because of usage, water and mineral deposits can seize the flush handle and not let the tank ball seat properly. By squirting a few drops of a lubricating oil on the mechanism just inside the tank and wriggling it, it should loosen the deposits and free up the handle. If it's rusted too badly then you will have to replace that part. The nut inside the tank is a left handed thread so keep this in mind before you start. It might save some frustration. To loosen turn the nut clockwise.