If you deleted your novel: please submit a help request! Be sure to include your username and the title of the novel.
If you deleted a chapter: click the circular arrow at the top of the “Chapters and Notes” in your writing space.
If you deleted words in a chapter: open the chapter, then click the circular arrow in the upper right left that says “restore chapter from archive.” Then click on whichever previous version of the chapter you want!
If you're not sure what happened but you're missing words: please submit a help request and include as much information as you can about what you were doing, what device you were using, and what you think might have happened.
Also, please remember to back up your work somewhere off the site every so often!