1. Do you see your carrier name and signal bars in the upper left on the screen or does it say "No service" or "No SIM card"?
2. Is your phone unlocked?
Here are some things to try:
- If it is not recognizing your SIM card, eject the SIM tray and reseat the card.
- Disable 3G and see if it works on Edge.
- Try a hard reset. Press the home and power buttons at the same time. Ignore the slide to power off and let go when you see the Apple logo.
- Reset your network settings. Settings > General > Reset > Reset Network Settings.