Because this is so unexpected and it impacts OpenLDAP installations, I though I share this information here.
Ubuntu server upgrade from 12.04 to 14.04 LTS will purge the OpenLDAP directory data. I don't understand why the or a upgrade would want to purge data but that is what happens. I can't find anything on the internet explaining why this is unavoidable and necessary.
From what I have learned so far the best way to recover from this is to completly remove OpenLDAP and start a fresh installation and import data from backup. I'm sure you will have it, for exactly this types of situations that are I'm afraid typical for OpenLDAP.
The reason is that it is unclear what state the upgrade leaves OpenLDAP in. I think it will take more time to figure out what was purged, what not, what was changed than it takes to do a complete reinstall.