1. Bad breaker
2. Dead short in the line somewhere
3. Bad electric heating element
You can test 1 & 2 pretty easily with a multimeter. #3 is what’s left when 1 & 2 check out ok.
Are you comfortable working with electricity?
Start by taking the wires off the water heater element at the tank. Keep them apart, not touching anything. Does the breaker stay on now? Should show 120V across those two wires.
Next you take the breaker out and remove the wire from the breaker. Put it back in (minus the wire) does the breaker stay on now? Should show 120V at the terminal the wire was on to ground.
You should have now isolated the problem to either 1, 2 or 3 and you can put everything else all back together.
Even without the electric it should still work on gas. You have to always make sure the tank is full of water before you turn on the electric or gas. Water will still come out the hot water faucet when the tank is by-passed, even though the tank is empty. With the water on, Partially open the blow off valve at the top of the water heater tank for a split second to be sure. Pressure or water should escape briefly when you do this. If not, the tank is not full.
