*** HOME button won't work all of a sudden - help!

I have a weird problem. My phone hasn't been dropped on the ground or in water but all of a sudden, the home button doesn't work anymore. Nothing feels broken on the phone (i.e. the button doesn't feel like it won't press down) but whenever I'm in an app and need to exist, pressing the home button does nothing! If I'm in my email and want to surf Safari, I have to reboot my phone - which is obviously very annoying.

I tried restoring my iPhone as both a new phone and from my latest backup but it still doesn't work - bummer. Is my phone broken or does anyone have any tips and tricks that could get it up and running again?

Thanks in advance for any help!