Are you experiencing a database upgrade loop when trying to upgrade from an older version of Shopp?
This is usually triggered by a caching plugin or you may need to add a constant to your wp-config.php file.
If you are using a caching plugin such as W3 Total Cache or WP Super Cache, then try clearing the cache and then clicking to continue the database upgrade.
If you are not using a caching plugin, then this may be related to a newer version of PHP (e.g. 5.5.x or above) and adding the following constant towards the middle of your wp-config.php file should correct the issue and allow the database upgrade to complete.
Login to your WordPress site using an SFTP or FTP client (e.g. Cyberduck, FileZilla, Transmit). Then browse to the WordPress root if you are not already in this location. This location has the wp-content, wp-admin, wp-includes folders and other files for WordPress. You should now see a wp-config.php file. Open that file for editing. Now copy the constant mentioned above and then paste it towards the middle of your wp-config.php file and save changes.
Now return to your WordPress dashboard and continue the upgrade again. Shopp should run the upgrade and your online store will be moved out of maintenance mode and reactivated afterwards.