The water heater in the rental property we recently purchased was not working. The house had sat empty for over a year and a half. We had someone out to inspect the HVAC and we asked them to look at the water heater as well. They advised against trying to fix it and would replace it for $1200, to which I replied, "um, no..."
Checked with my father-in-law who has his own rental empire, he said it was probably the thermocouple. I went to YouTube and watched a few videos, then went to Lowes and got a new one. Long story short, $7.75 later, the water heater is working fine and we've got hot water!