Helo Dar: David gave you great direction and a volt-ohm meter is one of the best investments a homeowner can make. Two other things come to mind. One is the dip tube. This plastic pipe delivers cold water to the bottom of the tank. Your heater might have cold water enter at the side/bottom, but it is worth a look. The other thing, which would be hard to do wrong in a single element heater is getting the wiring back as it should be. For instance, is the thermostat that got put in the right type. There are three to choose from. To elaborate on checking the element; with wires disconnected, put the meter in ohms times 1000 setting. Check from one screw to ground, which would be the element’s flange. You should get nothing on the meter… no movement of the pointer. If it moves, there is a leak to ground and the element is failing. Do the same thing with the other screw.
May you be enjoying hot water very soon 😎