Do you have any recharge active on the sim? Otherwise your service would be disconnected after 6 months without any valid recharge.
Are you connected via WiFi? Telstra/Boost has stopped their prepaid roaming arrangements for the medium term. But some people mention they can still connect via wifi for calls/sms (if your phone supports the Telstra VoLTE setting etc).
* I am a Boost Australia customer. This is a peer to peer community where users help each other.