Plumbing Questions? Ask a Plumbing Expert.
When this sort of things happen the most likely culprit is a bad lower thermostat or element. Residential electric water heaters have 2 elements (and corresponding thermostats) than are powered alternately. The upper is energized first then when that thermostat is satisfied power is sent to the lower. If the lower doesn't work then you essentially have reduced the capacity of the tank by about 1/2.
You first need to diagnose the problem. That requires doing some voltage testing. But if you don't have a multi-meter you could try replacing the lower thermostat IF you are comfortable working with electrical connections. If you are it's a pretty simple replacement and the thermostat is pretty inexpensive. Turn the power off to the heater then remove the lower access panel and the insulation behind it. Then you'll see the thermostat with a dial to adjust the temp. It is held in place by a metal clip and just slides up & out. Bring the thermostat with you to a big box store or a plumbing supply house to be sure you get the right one. They are pretty universal when it comes to replacements but the lower is different than the upper.
If replacing the lower doesn't solve the problem then you'll probably need a plumber
You're welcome. If you have a meter turn the power off, then remove the 2 leads from the element and test for continuity across the 2 terminals. If you have continuity then the element should be OK. Also check from each terminal (on the element) to the bare tank. You should read an open circuit.