This worked for me. Thanks Jparishll! Issue: My external Monitor stopped working after upgrading to BIG SUR. Computer Setup: I have a usb-c adapter connected which allows my external monitor , network, and I plugged my power in usb adapter and speaker to connect... Fix that worked for me: I first unplugged adapter unplug power from adapter reconnect adapt WITHOUT (Usb-c )power connected...FINALLY External monitor was found and displayed Then plugged in usb-c Power to adapter Possible Explanation: Some drivers might no be loading via adapters when Power is connected to it. Basically reconnect everything without power and then connect power to adapters...