Did you reset the modem after unpluging the power? If so, then try push the reset button reset the modem with power.
If that didn't work then contact optus to see if there is something stuck on the network because of the power outage.
A power surge when the power restored could damaged your modem. My cable internet modem was damaged, together with a few equipement connected via the Lan cable few years ago from a lightning storm.
sorry to hear about the charge, maybe reach out to technical support via livechat to see if they can assist with the charge. If it is caused by natural then optus might not cover the cost. good luck