This is a common complaint with the AC side of the water heater. Thankfully it is a very simple system to troubleshoot and repair.
The heater AC side plugs into a wall outlet near the water heater and goes directly to the ON/OFF switch. From there it goes to the hi-limit (top) bellows of the thermostat, through a connecting wire, to the low limit (set temperature - 140 degrees typically) bellows. then directly to the element then back to the plug.
Check for 110 volts at the bottom lug of the thermostat of a COLD water heater (with the AC switch on of course). If you have voltage there, the element is bad.
Remove the rubber cover that seals the thermostats (use Fonzie's picture) and examine the LEFT thermostat (black wires). If the wire that connects them is melted, is corroded, or the bellows, top or bottom, look bad, you will need to replace it. (Easy - just the 2 connectors and a nut).
eBay has them; just make sure you buy the one for AC. They are different.
If all looks good there, you may need to replace the element. Also within the capability of a handyman. Just make sure you buy the correct wattage AND 110 volts NOT 220 volts.
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