My site loses business by the day since it's been down for a week! Upgrade from 1.3.9 to 1.5.1 yeah right!
This is a stark example of why it is never recommended to try upgrades or modifications on a live site. If you had followed the advice given many places here, you would have done this on a *copy* of your site, with the live site humming along as usual while you took care of the upgrade at leisure in the background.

Your best bet if you can do it is to revert to the latest backup of files and database (you did take a backup before you started the upgrade, right?) so your store can get running again, and then work out the issues you are having.