There have been similar problems reported this weekend. If you have rebooted your phone, and still have the issue suggest you email PM directly from your account; just keep following Contact Us until you see Email Away. Make a note of your account number as it will be required in the email.
Other possibility is that you have not made a payment for 3 months. Does that apply to you?
because this is a prepaid service its treated a little different. Did you top up enough for what you need to pay for the month as well as has it been deduced from that balance? does it say active in your account? if yes to all this than try the usual reboot phone.
let us know if these are the cases and we can see if we can help more