Is this just on your Vox, or has it disappeared from your library on the Kobo website as well?
If on the Vox, swipe to the left page, which will take you to a "preview" page. There's a bug in the Vox software which causes it to place a "preview" banner on purchased books, and when that happens, the book will move off the "My Books" screen. Once you open the book and navigate to a point past the preview stop off, a refresh will remove the banner. Unfortunately, however, the problem tends to return when the Kobo software on the Vox is refreshed. Kobo staff is well aware of this and no permanent fix has been offered. I imagine that it will not be forthcoming now that the Vox replacement, the "Arc", is coming out. This is happening with a book I am currently reading, so losing the bookmark is annoying. I think I'll end up finishing my book on my iPad.