Happy New Year and Welcome to the Community!
Sorry to hear your wallet was stolen and you lost your SIM card. If you are concerned others might start using your services, you can report your device as lost or stolen via your My Account --> Usage & Services --> Report lost or stolen device (under Quick actions, near bottom of page). I understand your actual device might not have been stolen, but reporting it would put a halt on any usage of your services.
You can either go to a Fido Store to purchase a new SIM card or contact customer service to order another one. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Normally, you would be able to update your SIM card via your My Account (see here). However, you would need to have customer service update your SIM card since you no longer have your old SIM card number. You may need to update your SIM card in store with identification.
Hope this helps 😀