If your new to your carrier or just changed phones and it's not working at all. Don't panic, check the following before freaking out.

Is your ESN/MEID correct on the account? Is it the one from your new phone that is not working?
Does the ESN/MEID under the battery match what is actually in the phone?
Is all your NAM info correct? MDN, MIN, SID, NID, PRL, etc?
Is your account active?
Have you tried different PRL's?
Have you had the account refreshed?