If your internet connection is down or malfunctioning, a local credential and optional Bypass Code get you back into a Spriv-protected computer.
Disable Wi-Fi or physically disconnect the computer from the network.
Type in your local username and password as usual.
If a Bypass Code was configured during install, type it and click the arrow to proceed. If none was set, leave the bypass-offline password field blank and click the arrow.
You're logged in successfully.