What is wrong with our water heater?

In 1988 we bought a water heater it was called 'state'. In 1997, a flood came and we had to have it cleaned out and a new burner put inside of it. Now we are getting about 15 minutes of hot water. We have to wait for us to get hot water Any more and it only produces us less than 15 minutes of hot water... show more In 1988 we bought a water heater it was called 'state'.

In 1997, a flood came and we had to have it cleaned out and a new burner put inside of it.

Now we are getting about 15 minutes of hot water. We have to wait for us to get hot water Any more and it only produces us less than 15 minutes of hot water before the water turns cool again.

What should we do?
Update: From 1997 and there in out we had been getting perfect hot water. In 2017 was when the water begin to do this, it gives us 15 minutes of hot water how. Also we discovered the temperature gauge on the water heater is gone and missing and there is a little stick where the dial was attached onto it.
Update 2: no plumbing was done up until 1997
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