2016 theme dev site disappeared! Please help!

We have been working in dev (on a separate site) to update our current site to a 2016 theme (wide) and were planning to go live with the dev site on October 1st which would replace the current old-theme site. But I just went in to our dev site and it has somehow reverted back to the original theme and lost all of the changes made within the 2016 theme. Please help! Can someone help me figure out what has happened to our dev site? (Our current site which is live in original theme, is  beingwell.yale.edu).

Thank you,

Danielle

436-8270