Solution:
You should edit your wp-config.php and add
define('WP_DEBUG', true);
so that you can see the errors on your screen, at least until you figure out which plugins are causing the problems.
If it is a matter of PHP version, check with your host to see if they can roll you back, or you’ll have to either contact the plugin author and have them update the plugins, or you need to fix the code yourself, based on whatever the error messages you see are.
Edit: if this doesn’t solve your problem, first try a default WordPress theme like twenty-nineteen or whatever. If that doesn’t solve it, then disable each plugin one at a time until you find the culprit.