i am using iphone 3gs and my wifii button is greyed out!! please help!!

Here's the troubleshooting procedure:

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

Did you really try all five steps in the article?

Then as it says in the article right after the five steps:

If you are still unable to turn Wi-Fi or Bluetooth on or off after attempting these steps, contact Apple or your wireless carrier for additional assistance.

Here's how to contact your wireless carrier:

Contact a mobile carrier

This often is due to a hardware failure in the phone, and can be fixed only by replacement.