Sequoia upgrade USB-C/Thunderbolt connection

If your external monitor (via USB-C) stops working after a Sequoia upgrade try changing the "System Setting>Privacy & Security>Allow accessories to connect>accessories"

Always Ask (more than likely the culprit)

Ask for new accessories

Automatically allow when unlocked

Always allow

The 3rd and 4th options should find your USB-C device including your external monitor

Thanks to Jim in Dublin for the hint.