euca-describe-nodes to verify if instance is there. Is the instance there?
Yes: Go to the NC log for that NC and grep your instance ID. Did you find the instance?
No: Go to the CC log and grep the instance ID. Is it there error message?
Yes: The error message should give you some helpful information.
No: grep the instance ID in cloud-output.log . Is there error message?
No: Log in as admin and run
euca-describe-instance . Is the instance there?