If you lose your .pem file you will need to log on to the Yoti dashboard and regenerate a new key.
To do this please navigate to: https://www.yoti.com/dashboard/
Find your Yoti application and navigate to the key tab. Press the re-generate key.
This will download a new key which you will need to add to your development project.
Please be aware, once you generate a new key your integration will stop functioning until you deploy your new key pair.