Check the list on the System Status page. If the box by iOS Device Activation isn't green, try to activate your iPhone later.
If asked to enter your password as you activate your iPhone, enter the password. (A password prompt means that Activation Lock might be enabled in Find My iPhone.)
Restart your iPhone.
If you're using a cellular data connection, connect to a reliable Wi-Fi network instead.
If you still can't activate, here's how to activate your iPhone through iTunes instead:
Make sure that you have the latest version of iTunes.
Check that your computer has an Internet connection.
Open iTunes on your computer. With the cable that came with your iPhone, connect your iPhone to your computer.
Wait while iTunes detects and activates your iPhone:
If iTunes shows, "Set up as New" or "Restore from Backup," it means that iTunes activated your iPhone.
If an error in iTunes says that the SIM card isn't compatible, contact your carrier.