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If the last recharge ran out more than 6 months ago then the SIM will have been deactivated and you will need to get a new Mobile Broadband Starter Pack.
If it is just that you are on Prepaid Mobile Broadband and the recharge has run out, but the SIM is still active, then you need to log in to My Account via the 24x7 link on the website and recharge it. If you are having problems with doing that, then a trip in to your local Telstra Store would be the quickest way to get it resolved.
@Jupiter Thanks for the info, I wasn't sure on the life expectancy of an inactive SIM.