Many of my customers ask me why after upgrade a new version, their online store is working fine but the Admin Login is broken. Some store has front-end broken due to heavy customization. It just displays a blank page. So, this is what you can do to check and re-config everytime you upgrade. It doesn’t guarantee that it’ll work but these are the minimal things you must check before modifying any codes and logic.
1)  If you’ve heavily customized lot of codes and product attributes modules on your site, make sure all those codes don’t get overwritten by Zen-Cart upgrade.
2)  Go to YOUR-ADMIN-DIR/includes/ directory. Open Read+Write permission to the file called configure.php. Open it in an editor and start changing some parameters/values that are applied to your server environment.
3)  Add a new file called test.php in the root. Type this codes:

<?php
echo $_SERVER['DOCUMENT_ROOT'];
?>

Run this file in browser. Then use this value to replace in your configure.php file. Make sure the store reads the correct path on your (new) server.
4)  Make sure all tables in MySQL Database are not corrupted during data import by opening each table to review data and structure.
5)  Run MySQL query to compare the old and new tables in Database. Make sure they both have the same number of tables. Some may get lost during data transfer, who knows.
SQL Query:

SELECT COUNT(*) FROM information_schema.TABLES WHERE TABLE_SCHEMA='ZenCartStore'

If you found the mismatch, try to import data again or just import those missing tables.
6) Remove all sessions in database to make sure that all old sessions using old encryption method are gone.
7)  Check configuration table in database to make sure all old & new config keys + values are there.
Finally, if it’s still not working, please DO NOT modify any codes if you don’t know the ZenCart functions and logic. I’d recommend to hire ZenCart or at least PHPMySQL developer to look at it and fix it. Once you upgrade, you can’t revert if you didn’t back up database and your file system before upgrading.