All the magic lies in the cache directory, the debug files found there will point you right at whatever the problem is. Additional thought, if you can get to the admin, but no store, might be worth taking a peek at the includes/configure.php file.....ANYTHING wrong there will stop everything, cache file directory path, store URL, SSL issues, et.all.....
I muddled thru getting this up, tested, and configured, and we are up and running on 1.5 with every single mod we were using with 1.39h......
For me, once I backed up, and started paying more attention to what I was doing, things went fine. Having learned my lesson in upgrades past, I did choose to install full 1.5, get that running as advertised then started brining in mods......just seems easier than all the winmerging, and prob. a bit safer......and lastly, I do avoid mods that mess with core files....not that I don't use any, but am picky
IMHO, the entire key to setting up a Zen-Cart store is first install/run it out of the box, then mod it......
Good luck!
Dave