You will have to give us more information before we can answer that.
I expect that your problem is that the APN (Access Point Names) settings are missing or incorrect. You can find these in the mobile network settings. What values they need will depend on what network you are on, which is why I can't answer this without more information. However, you should be able to obtain these from your carrier - most likely they are on your carrier's website - or by searching the internet for your network name plus APN settings.