Time Capsule will not connect to the internet or back up
I would start with the Airport Quick Assist and see which steps fail you.

http://support.apple.com/kb/HT1145

You can leave this question a general. "How do I start to troubleshoot my device" or edit it to list the steps you took and the specific error conditions. I'd be inclined to start a new question once you have more details and have narrowed down the issue.