Craft 3 forgets plugins
Craft 3 knows about available plugins via the vendor/craftcms/plugins.php file, which stores info about all Composer packages that were installed with type: craft-plugin in composer.json.

If that file doesn’t exist or is just returning an empty array, then make sure that the craftcms/plugin-installer Composer package is installed, and try re-requiring each of your Craft plugins. As they are (re-)installed, they should get added to plugins.php.