What is your status?
Can you restart your phone?
If it didn't work Can you try sim in different phone?
Are you in the US currently? Make sure your roaming is enabled and data too if you have a data package.
@1433 - Did you enable roaming on your phone? Is your mobile network set to automatic?
EDIT: PM uses AT&T & T-Mobile in the US.
Make sure you add and purchase the US roaming add-on. Also check your account under add-ons to make sure it is showing. Also you need to enable roaming.