Hi, think your plugin is not compatible with the latest illustrator you should check on that webpage...regards
It was working up until last night.
We are sorry for the trouble. I would request you to please try the steps shared in this article (https://helpx.adobe.com/illustrator/kb/plugin-missing-issue.html). However, if you are using a third-party plugin, then please check with the manufacturer if they have made it compatible with v26.3.1 or not. In the meantime, downgrade the Illustrator to an older version so the plugin can be used until the manufacturer release an update.
Kindly update the discussion if you still need assistance with it. We'd be happy to help.
Thanks & Regards,