Is the file still large enough to reasonably assume all contents is still in there? ("1Kb", for example, is not.)
Is the file small enough to post somewhere on-line so we can take a look at it?
Have an indesign book file that all of a sudden contains no pages after saving and closing.
The "book file" (.indb) won't have any pages in it. It does, however, contain a list of where all of the chapter files are - and the chapters are .indd files, which I assume contain your 30 hours of work.
Open up your .indb, and then when the Book pane pops open, you can doubleclick on your chapters there to open them up. However, if you moved those chapter files yourself (to another folder or drive), the book file won't be able to find them. So it's important to leave them in the same location for the duration of the project.