This is a common problem, but also one that can be easily solved. The possible issues are:
Wrongly inserted SIM card - Just put the SIM card so that the golden connectors are facing upwards and the little plastic cutout facing away from the phone. After doing that, power your Ringo off and on again.
The SIM card is not activated - if the SIM card is new, it is possible that it still hasn’t been activated. Check the provider’s instructions on how to activate the SIM card. It’s usually done with a simple phone call or a text message.