You probably have 2 heating elements. One top and one bottom.
First feel the outside of the tank when the water is fully heated. Does the bottom feel much colder? If so, an educated guess is the bottom element is gone. If not, possibly the top. This is just an educated guess. If it is the bottom element you will have to drain the tank completely.
1. TURN OFF THE POWER AND TEST TO MAKE SURE IT IS OFF
2. Drain the tank
3. Remove the wires to the suspected element and test it for continuity. Replace the element if necessary. If the first element tests OK, you should check the other element.
4. Reconnect everything and completely fill the tank before turning the power back on.