I am in urgent need for assistance.

I have been called in to assist a company who have mistakedly upgraded their Ubuntu version from 10.04 to 12.04.
So therefore their Zimbra is not not loading/starting.
I am unsure of the process to fix this for them.
I believe I have to get the system back to the old version of Ubuntu and reinstall ZCS.
They have a full backup but I am unsure as to exactly what this has backed up.
I have read you need a copy of the /opt/zimbra structure.(not sure exactly what is meant here)
If someone could reply ASAP it would be greatly appreciated.
My knowledge is poor in this field so please bear with me if I ask stupid questions.