why is my lawn going brown

Some lawn treatments need water soon after application, either by rain or hosepipe.
Maybe double check the directions on the packet because i know this has happened to a couple of people near me ..... they hadn't watered afterwards.

and/or maybe the concentration was to strong. To much feed can cause juats as many problems as none

If you applied the feed and then we didn't have rain, and recently, the very hot weather, then that's the explanation. Water it thoroughly now, if you've not had enough rain wherever you are.