Okidokey.. since youve been running this water heater for a few years, and it is only now acting up... my guess is that one of the heating elements has gone bad. A second place to check is the thermostat controlling the element. (usually one thermostat per element, 2 elements in a standard water heater).
A voltmeter is the quickest way to check these parts. If the thermostat has gone bad, then it will not turn on the heating element... no matter what temp it is set to (most thermostats have a tiny dial on their face, you can set with a flat blade screwdriver).
If the thermostat is good, and it is the element that has burned out, then you will have power going TO the