Could not scan for wireless network

Try Resetting the iPhone

Reset the iPhone by holding the sleep/wake button at the top right of the device and the home button at the bottom center of the face at the same time. Hold them both until you see the screen go black and the white Apple logo appear. When this happens, you can let go - the iPhone is rebooting.

Last idea would be to try where there is public wifi, Starbucks, Burger King, etc... and if still can't scan or see, then make an appointment with the Apple Genius bar and you may wind up with a replacement phone if still under warranty.

I have the same problem with my iphone 3G running OS 4.2.1. Could not scan for wireless network. I tried all the reset network settings (which resulted in Graying out of Wi-fi and "No Wi-fi" in settings). Then I followed the instructions to hard and soft boot the phone using the home key and the power button, that didn't work either. Next I resorted to the restore of the phone, that didn't work either. I turned on the Airplane settings, powered off the phone and turned it back on then turned off Airplane settings and the Wi-fi became available again. Unfortunately I'm back to the Could not scan for wireless network error message after trying to join my home wi-fi. It used to work prior to the OS 4.2.1 upgrade.