You should easily be able to hear the burner turn on. If it isn’t turning on, that limits your problem areas.

If it is not turning on there is only one other thing I can think of, which is the flame sensor. Basically look at the pilot flame, there should be a metal rod in the flame. That is a sensor designed to heat up trip a signal. If that signal isn’t tripped, the system can’t run.

It is a very common issue. Super cheap part.

You can diy fix by either replacing or taking some sandpaper to it if it looks scorched. Sometimes it builds enough carbon on it to insulate it and not allow the main flame to come on. Either way 5 minutes work, maybe 15 if you really don’t know what you are doing. Does require turning off the pilot.

You should take the opportunity to flush the tank as well. That’s simply just draining the water through the hose bib at the bottom. Sediment builds up an can be an issue. Should be done once a year. Must be done with the flame off though.

Procedure to do that is turn off the incoming and outgoing shutoffs. Turn off the pilot. Open the pressure valve (weird lever thing on the overflow pipe. Make it stick straight out). Then attach a hose to the bib and lead it to a drain in the floor. Open the bib. It will take an hour or so to drain and it will pull the sediment at the bottom with it.

Once done turned on the supply to get s few more gallons in the bottom and drain that. Just to get everything out. Then close the bib and turn on the supply. Fill until a bit of water comes out the overflow and close the over pressure valve. Then give it about an hour to settle and open the shutoff connecting the system to the hot water supply.

The hour to settle is really important. Otherwise you will get bits of calcium and the like in your faucets.

This process should be done once a year in most areas with medium to hard water.