It's hard to tell what happened to be able to give a direct answer.
However, Pages has a built-in versioning system that's built off Time Machine. If you have Time Machine configured on your machine then it's easy to scroll through previous versions and go back to the 35-page version.
Use File -> Revert to... -> Browse All Versions
Hopefully (if Time Machine is running), you'll see a Time Machine interface with your active document on the left, and historical versions on the right. You can scroll the right hand list to go back in time. Once you find the version you want just click Restore.
If you don't have Time Machine running, your options are limited, though.