If site is not displaying properly before/after installation or updating and instead you can see a generic server error similar to image below or just a loading spinner, you might need to apply one of below solutions depending on your hosting.

Some hosting providers have strict rules for permissions on files and folders. You might need to manually change "public" folder permissions to 755 and "public/index.php" file permissions to 644 via file manager.
Sometimes default .htaccess file in the main directory and/or "public" sub-folder might get overwritten by your server or not be uploaded at all. You will need to manually open these files via file manager and verify that their contents are correct. You can find htaccess.example file containing original content right next to both .htaccess files.
Sometimes you might need to make a small change to one or both .htaccess file mentioned above. Open the file with text editor and uncomment (remove # symbols before it) this line: "RewriteBase /" This is explained in more detail in the .htaccess file itself.
Apache mod_write module might be disabled on your server/hosting. You will need to enable that module in order to fix this issue. It can usually be enabled from cpanel, but you might need to contact your hosting support and ask them to enable it for you.
If solutions above don't fix the issue, you can open "errors" page in your cpanel to get more information on what is causing that issue. If creating a new ticket regarding this, make sure you include that error log along with the ticket.