I can't get my iphone4 to connect to any wi-fi

Reboot your router by turning it off for 10-20 seconds.

I have tried, my sisters phone is fine its just mine that wont connect and no i cant even turn it on its gone grey

Try this: http://support.apple.com/kb/TS1559

I just have to tell you, I had massive problems with my 4S in regards to connecting to home Wi-Fi. The phones setting for this was always greyed out. I saw Optus about this, restored the phone a million and one times. The phone would not even connect pass the itunes log on.

FINALLY, I restored the iphone for the very last time - at least I was able to get passed the itunes logo. But alas, still unable to connect the the home wi-fi system. I did some more googling and saw a post regarding placing your phone in the freezer for 15 minutes... yeh right, but as a last resort I thought Id try it. Turned the phone off, placed it in a zip lock bag, set the alarm - OMG - it worked!!! I cant believe it. I am usually a cynic about all these posts etc, this is the first time making a post in my life. I am amazed this actually worked!!