While we certainly hope that this doesn't happen to anyone, if you do find yourself in this situation, please let the front desk know. They will be able to issue you a plastic key and deactivate your mobile key.
We always recommend to our users to utilize the built-in security features OpenKey offers (Touch ID/Face ID) in addition to their device's native security features like passwords, passcodes, or biometrics to limit access to their mobile phone.