Hey Lucy,
It might be the case that the emails are broken away from the template by editing the code directly in the email editor. You can check to see if the email is still associated with the template by finding the email asset in the file tree (Design Studio or Marketing Activities), clicking on it (to bring up the details in the right panel) and then looking at the "Template" field list there. If there isn't a template name, the asset is no longer associated with a template.
This usually happens when you go into the "Edit Code" part of the email editor and make changes outside of the editable areas and modules there. These edits instead should happen in the template and then be reapproved in the email asset. You'll need to drag in a "new version" of the modules you've updated in the template and copy/paste over the content from a previous example when you make changes to the modules.
To get the modules tab back to being activated, you can start a new email based on whatever template you were using and avoid using the Edit Code functionality which will break the email from the template (there's a little warning at the top of the Edit Code dialog box with a warning to this effect).
Here's a link to the Marketo Docs on this functionality, you'll notice the warning at the top of the screenshot here, to the left of the SEARCH CODE orange button:
https://docs.marketo.com/display/public/DOCS/Edit+an+Email%27s+HTML#EditanEmail'sHTML-Makeanychanges...
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