How do you make open files appear in tabs?

Hi there,

Sorry to hear about the trouble you are facing. In addition to @tromboniator suggestion, if you are on Mac, you may try Checking "Application Frame" is turned on in the Windows menu. If our suggestions don't help resolve the issue, then you may try resetting Illustrator Preferences ( https://helpx.adobe.com/illustrator/using/setting-preferences.html) and let us know how it goes. I hope it helps!
Disclaimer: Please note that resetting preferences will remove all the custom settings, and Illustrator will launch with default settings. You can also take a backup of the folders in case you want to. Location is mentioned in the article.

Thanks & Regards,

Anshul Saini

You should be able to drag the title bar of one open document onto the title bar of another, and they will join as tabs. To open documents as tabs go to Preferences > User Interface > Open Documents as Tabs.

Peter