Your site will be scheduled for deletion if nobody has signed into it for 12 months and it has little traffic.
To unschedule the deletion, you can do so from MY ACCOUNT. Unschedule next to the site name - Click on the green button next to "Cancel Site Deletion"
Afterwards go into ADMIN HOME for the site so it will be classed as someone recently signing in.
Often people setup more than one site when trying our system so it is worth checking the URL of the site in the email, it may be an unused test site.
If your site comes up - it is not scheduled for deletion.
If you go to your site and see a page like this then it has been scheduled for deletion