what to do if all my adio books are missing

Audiobooks are a one-time only download from the store. If you no longer have them in your phone's Music app then have you got copies of them in the Books part of your computer's iTunes library (had you copied them off your phone via File > Devices > Transfer Purchases ?) and/or on your backup of your downloads/library ? If not, and if you haven't changed countries since buying them and they are still in your country's store, then you could try contacting iTunes Support and see if they will grant you a redownload : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page

But if you aren't already keeping and maintaining a backup of all of your downloads then you should be doing so, any item (apps, ibooks, an album etc) could be removed from the store at any time.