If you activate the Tag Groups plugin and don’t see your tags in the WordPress admin area, you don’t need to worry. The tags are still there, they are just not visible.
This problem is normally caused by the filter menu on top of the “Tags” screen. Try selecting “Filter Off” and click “Apply”.
You can also try going to “Tag Groups”, then “Back End” tab and disabling the filters.
You can also go to “Tag Groups”, then “Tools”, then the “First Aid” tab. Click the “Reset Tag Filter” button. That may bring the tags back.
Finally, test for plugin conflicts. Deactivate all other plugin one-by-one and ccheck if now the filter appeared on top of the tag lists. The filter might fail to show because another plugin is causing an error on that page.
To get more details on the plugin causing issues, open the console in your browser, as described here. Then reload the tags page and check for error messages in the console. The messages usually point to the plugin that is causing the error.