Welcome to the community!
Sorry to hear you've lost your phone. Had you had a chance to report your device as lost with Fido? You can do so via My Account --> Usage & Services --> Mobile --> Quick Actions (near bottom of page) --> Report lost or stolen device.
You can go to any Fido store to get a new SIM. You may also contact customer service or send @FidoSolutions a PM to have a new SIM sent to you. While the Quick Actions (from above) also has an option to Update SIM, since you've lost your old SIM, you'll likely require assistance from customer service in order to update your account with the new SIM card. You may need to provide identification so they can verify you are the account holder.
Hope this helps 😀