Generally that would mean that the service had not been recharged within 6 months of the expiry date of the last one. If numbers for inactive for that amount of time they get deactivated and put back into the number pool.
You could check with the prepaid team to confirm (125 8888) but I'd say you'll need to purchase a new SIM starter kit and get a new number
I am a Premium Services Support Consultant (Telstra Platinum, Telstra Smart Home, Locator/Device Locator and Neto POS). However, I am not an official representative on CrowdSupport.