How do I adjust my hotwater?

A problem at our house was that the water was extremely hot(not a prob cus i loved it). Someone came to visit and decided to adjust the hot water to conserve oil. The sinks still have hot water but our shower only stays lukewarm for 1 minute before going practicall cold. The only way it will heat is if i flush the toilet over and over. thats stupid. Does anyone have any clue what was done to do this so that I can undo it. If we can't figure it out we'll call someone, but for the sake of saving money if suggesting a plumber is the best answer you can give than don't give one.