My iPhone/iPad says "No Service" with any SIM

Probably you installed ultrasn0w on an incompatible baseband. Open Settings, click on General, then on About and scroll down to Modem Firmware to check the version. Ultrasn0w is compatible with these versions, depending on your device: If you have any other baseband version, go to Cydia and uninstall ultrasn0w. If it still doesn't work, then you need to restore your device. Afterwards read about how you can unlock (if at all).

Another reason for this could be that your SIM is broken or has expired. Usually prepaid SIM cards expire after a few months when not in use. Test the SIM in another phone first.

A third reason could be a hardware fault. This is not covered by this site.
last edit: 2011-01-12

Most frequently read after this
Most read articles overall
Recently added or changed
Keywords
 iPhoneiPod touchiPodiPad
 Apple TV 2GjailbreakunlockiOS 4.2.1
 Jailbreak FAQJailbreak F.A.Q.iPhone 3GApple
 mobile phonefirmwareiPhone 4iPhone 3GS
 howservicedfusim
 whatsaysmodecarrier
 recoverybasebandlockedmodel