The most common reasons that a webpage stops working after an update is that your CMS is not updated to latest version.
To update it, you need to log in to your admin panel and upgrade your CMS.
Another reason often is that plugins or themes do not support the new update.
It is most oftenly solved by updating the plugins ot themes that is causing the error.
You need to turn to each developer of plugin to see so that the exact plugin you´re using supports the update you make with your CMS.
Typically speaking, the most common case is that there is version history you can look in to.
If your website becomes completely white, it is usually becuase an error message is generated, but no error display is turned on.
If you use WordPress (Most common CMS), you need to log in via your FTP account.
Here you shall choose to edit the file "wp-config.php", and set the value "wp_debug" to "true".
Your website will then display an error message. After that you can deactivate the plugin which give you the error message.
You can also turn on error messages by doing as below, for example if you do not use WordPress:
Create a file called "php.ini" and save this file directly in the root directory "public_html".
Add the following in the file "php.ini":
display_errors = 1;
display_startup_errors = 1;
error_reporting = E_ALL;