Hello Nup,
Welcome to the community!
Sorry to hear you've lost your SIM card. Not to worry, though, your phone number is associated with your line. Updating your account with a new SIM card would not affect your account or phone number, only how your phone associates with your account.
Had you reported your SIM card as lost to Fido? You might consider doing so as it would prevent unauthorised usage on your account (see here).
With regards to obtaining a new SIM card, you can either go to a Fido Store to purchase a new one or contact customer service.
Normally, you would be able to update your SIM card via your My Account (see here). However, you would likely need to have customer service update your SIM card since you no longer have your old SIM card number. Part of the process to update the SIM card on your file would be to receive a verification text to your current SIM card (physical or eSIM). The requirement to receive the verification text to your old SIM card is to prevent SIM-swap fraud (see also here etc). Since you no longer have your old SIM card for the verification, you may likely need to go to a store with adequate identification in order to activate the new SIM card.
Hope this helps 😀
Cheers
