Have you tried to close - using the apple + q keyboard shortcut. Then Restart the app, does the close button come back?
What version of Illustrator? You can trash the prefs for Illustrator and then reopen the app and it should be fixed.
I would first try quitting/force quitting the app and restarting your computer.
Twist
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Everything will be OK in the end.
If it's not OK, then it's not the end