Top wake/sleep button not working (iPhone 4)

This discussion is about the iPhone 4 on 3 mobile.

Hi

As an introduction, my name is Thomas.

On my iPhone 4, my phone was working fine, but all of a sudden, yesterday my wake/sleep button on the top of my iPhone decided to not pop up after I pressed it to go into Sleep. Luckily, my home screen button isn't broken, so to wake it all I need to do is press the home symbol, but when putting into sleep mode, I have to wait a minute of time I don't have to wait for it to go off.

Please can someone tell me what to do!

Thomas