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The problem is not with the backup archive, or Kickstart, but with the host.

Since you're in a time crunch, I am not going to guide you through troubleshooting your server issues but rather tell you how to get the site restored. After all, this is the only reason I am replying to you after hours (it's past 10pm where I live). Important to remember is that Kickstart only extracts the backup archive, it DOES NOT restore your site. The actual restoration uses a script that is included inside the backup archive itself and gets extracted in the installation directory.

Set up a local web server on your computer e.g. MAMP, or WAMPserver.

Put Kickstart and a copy of the backup archive file(s) in the web root provided by the local server, and run Kickstart locally.

When you see the "Run the installer" button, close your browser.

Go to your local web root. Delete Kickstart and the backup archive files.

Upload everything else into your live site's web root.

Now visit your live site's /installation/index.php URL to complete the restoration.

At the end of the restoration process the restoration script will attempt to delete the installation directory. If it cannot do it, it will let you know; just delete that directory yourself using (S)FTP or your hosting control panel's file manager.

As it happens, I am still awake :D

Try deleting the administrator/cache/autoload_psr4.php file. This should force Joomla to rebuild its autoloader cache. This is the first thing you should always do when you get class not found errors on your site.

Regarding the DB restoration, I believe it's linked to the problem you had with extracting the backup archive. You should be able to work around that by making a few changes in the database restoration page:

• Use REPLACE instead of INSERT: enabled
• Stop on CREATE error: disabled
• Stop on other error: disabled

A server crash is impossible to diagnose without much, if any, information to go by. From what I understand, you restored the site on the exact same server. You did not change servers, and you did not do a server rebuild. This rules out a hardware issue, or a server configuration issue. Therefore, it is more likely that it was a problem with your site, not the server. I can't tell you what it was without having the exact error it was showing, and without knowing what actions were taken right before the site stopped working properly.

The correct process in this case is to edit the configuration.php file to set $error_reporting = 'maximum' and $debug = true. This makes Joomla display the debug page when there's an error, with very detailed information about what happened. This is what a developer like me uses to start troubleshooting a site that no longer responds. If that makes no change, we go one step back and replace the contents of the .htaccess with the default content from Joomla's htaccess.txt, and then select the correct PHP version in the server's hosting control panel (since it might change the .htaccess file to apply it). Between these two simple actions we can start troubleshooting 90% of site issues. The other 10% of problems are not exactly crashes but misconfigurations in the server itself, DNS, any CDNs, server and site protection software and so on. These are easier to identify (they happened after making a change to one of these items) and harder to troubleshoot (you need experience with the particular software or service that has a misconfiguration).

Yes, I do realise that this is a long-winded way of saying "it really depends, mate", but it's also the objective truth. This is where experience and intuition come into play.