Sophia here from Shopify. I’m so sorry to hear that you received a chargeback.
You are correct that your store needs to be on an active plan in order to access the admin and review the chargeback information. You can reopen your store by logging in and selecting a plan. However, Shopify does automatically submit some evidence for the chargeback on your behalf, such as product details, the shipping company used (and tracking information if applicable), the date the order was placed, and the date and time of order fulfillment.
If you don’t have additional evidence to submit based on the recommendations outlined in the Chargebacks and inquiries document, you don’t necessarily need to log in and submit evidence.
Do you know if the order was received, or have you had any contact with the customer?