By default the document library is stored in a filesystem: Restore it
from backup as well (if you've used default values, it's in liferay's
data/ directory)
Note: Sometimes it's just the directory where the document library is
expected: Setup-Wizard configures ${liferay.home} - and if you've
changed the location where Liferay is deployed, the document library's
content might still be expected in "the old" location.
That's yet another reminder for my standard wisdom: "You're only
allowed to call a backup a Backup, if you have recently demonstrated
that you can use it to restore a working system on a completely
unrelated environment". This should be one of the first
operations, and then repeated from time to time.
Your log file will also tell you which file is missing.
