I have been using my Mac desktop as a second monitor with my MacBook Pro for a few months now. All of sudden my Mac wont show up in screen mirror. What happened?

Sidecar is for using an iPad as a display for a Mac. Not for using a Mac as a display for another Mac.

Maybe you meant AirPlay to Mac?

Set up your Mac to be an AirPlay Receiver - Apple Support

System requirements for Continuity features on Apple devices - Apple Support

A 2019 iMac or 2019 Mac Pro, running Monterey or later, could be an AirPlay Receiver. But you would need to turn on that feature in the System {Preferences/Settings} for that Mac.

A careful reading of the article linked to your post:

Use your iMac as a display with target display mode

"The iMac used as a display must have macOS High Sierra or earlier installed."

"The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed."

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if you stepped outside that envelope, using an iMac in this way is NOT SUPPORTED.

Not supported means, if you CAN get it to work, "Merry Christmas!" You get an unexpected present.

Not supported means, If you can NOT get it to work, no additional help is available.

what Model-year Mac and other device(s)?

what old version MacOS?

what new version MacOS?

Use an iPad as a second display for a Mac - Apple Support