Question
Why would just part of my lawn turn brown, even though I water it every day and fertilize it on occasion and what can I do to make it green again?

Answer
Whatever has turned brown will never turn green again but new (green) growth can take place. From what you write, the problem can be environmental. Was a vehicle/boat parked there for any length of time? Did a spill of some sort happen? If so, replace the soil and resod. Water may be a problem only if a part of the sprinkler has failed. Check the system. If it is a low area on the lawn, water may be sitting there too long and suffocating the grass roots. Gradually filling in this area will take care of that. If these two are ruled out, then over application of fertilizer can cause 'burning'. Could this have happened? Then resodding the area is a simple solution. Then there are insects. Soap flush around the area to see what comes up. Identify all and if any are harmful, then apply an insecticide.
If all these are ruled out, then a fungus is present. This will require identification and then a treatment recommendation. Contact your County Extension Agent on this one.