Im sorry to hear that. Boost SIMS stay active for 6 months without being recharged. If it has been longer than 6 months and you have not recharged your SIM, it will become inactive. However if the SIM is inactive you should not be charged.
Please contact Boost on 125 8881 (8am-9pm, 7 days) or on live chat (24hrs, 7 days) so we can check the charges get this resolved for you.