Disconnect the chain from the lever,and using a pair of pliers turn the nut on the lever counter clockwise. These threads are reverse from the norm.

That's how it is supposed to work. Pulling the lever lifts the flapper in the tank which lets water into the bowl.

Assuming you have a regular tank toilet with a ball float and rubber flap and that the water is turned on and the tank is filling, the most common cause of a toilet not flushing is a broken or detached chain that runs from the handle lever to te rubber flap that keeps the water in the tank.

They are usually about 24-26 inches high

It lifts the tank flapper /bulb connected to the Douglas Valve

On a Flushometer it moves the piston assemble aside