Ack! Big problem...

My site goes down every time I do something in my admin. If I restore the files from the previous day, all is well again. But then if I even look at something in my admin, the whole site goes down again. I have tried to upgrade my database with the zc_install but I get the following:

Problem Detected

Sorry, a problem has been detected.

This installer needs to be able to start a PHP session in order to complete installation.

I don't think it's a timeout problem. The cache folder permissions are right. And my host accepts php. I have been adding on mods recently and I think one of them messed me up, but I didn't discover this problem until it was too late to restore my database...it's been too long and the restoreable databases aren't old enough to roll this back. Please help! What did I do????

I have been adding on mods recently and I think one of them messed me up, but I didn't discover this problem until it was too late to restore my database...it's been too long and the restoreable databases aren't old enough to roll this back. Please help! What did I do?
A list of what you added in what order might assist along with what ver of Zen Cart, the type of server 'nix or win, etc...

BTW: Do you not maintain your own local DB backups?

Well that was one of the mods I was trying to add was the backup from admin. I think what broke it was when I was trying to install links manager and I deleted old tables from the previous version and that's about the time things went kerplooey. So that's why I'm trying to upgrade my database, to try and fix what I boogered up.