Lost Art Board. Help!

It's probably not just the artboard that is gone, but also the artwork?

Did Illustrator crash when you last saved the file?

Was that the artwork you created in your last session?

Where do you save your work? Or is it a Cloud file?

You probably can't recover the file. What you can do is turn off "Save in background" in the preferences, because this is one of the issues it can cause.