External display doesn't dectect

Are you using any third-party software (DisplayLink) that connects a monitor to the USB instead of standard display ports (DisplayPort, Thunderbolt)? That software may not be compatible with Big Sur.

If using a standard display port, have you tried restarting with the display attached?

I meant restart your Mac with the display attached.

What port are you connecting to--Thunderbolt, DisplayPort, HDMI? Have you tried a different port or cable?

Does it appear connected in System Information under the section for the port you are using?

Maybe an SMC Reset.

I have a Dock which plugs into the two USB-C ports and has an HDMI port. That works without issue on Big Sur, so I wouldn’t think it is Big Sur unless it is a compatibility issue with the hardware or software.

The USB-C port can push USB, DisplayPort, or Thunderbolt. There are monitors which connect over USB. Apple doesn’t support that. But, if your adaptor worked before, it is likely a DisplayPort to HDMI. If it is USB to HDMI, you would have needed DisplayLink software to make that work.

I don’t think any Mac pushes HDMI over the USB-C port. You would need an adaptor that converts from Thunderbolt or DisplayPort to HDMI. But, I couldn’t say for sure.