My project disappeared.

Your books are saved in the browser you created them in, so a book "disappears" when that browser's storage isn't the one you're looking at. (If you backed it up with the ☁ button, sign in first — it may simply be waiting in your account.) Run through this:

• Same browser and device? Open My Books in the exact browser (Chrome, Safari…) and computer you used before. A book made in Chrome won't show in Safari, or on another machine.
• Did you clear browsing data? Clearing cache/site data erases locally stored books. See What happens if I clear my browser data?
• Private/incognito window? Those discard everything on close.

To recover:

• If you ever downloaded the project file, restore it with My Books → Open project file… — it brings the whole book back, manuscript included.
• If you purchased, sign in and your license keys sync back so you won't pay again (you may need to rebuild the project itself).

Going forward, download a project-file backup — it's the reliable safety net.