First, make sure it's really deleted and not just in the Trash:

1. As an Administrator, go to the domain that should contain the section
2. Click on the Manage tab, then Trash
3. If you see your Sections root then select it and choose Restore

If it's really deleted, then you can use Nuxeo Shell to recreate it (note that before Nuxeo 5.4.2, the Nuxeo Shell was only available as a separate download, since Nuxeo 5.4.2 it's available directly in the Admin Center).

1. Go to the Admin Center, click Monitoring then Shell
2. If not already connected (when using the shell from the command line), connect to your Nuxeo instance: connect http://myserver:8080/nuxeo/site/automation/ -u Administrator -p Administrator
3. Go to the domain in which you want to recreate the section: cd /default-domain
4. Switch to Automation mode: use automation
5. Create the document: Document.Create -type SectionRoot -name sections
6. Go back to the Document Management tab; you may need to refresh the left-hand tree or the right-and listings using the refresh buttons.
7. You can then give sections a proper title like “Sections” by editing it.

See https://doc.nuxeo.com/x/E4dH for more about Nuxeo Shell, and https://doc.nuxeo.com/x/JIdH for the Automation commands (here we used Document.Create).