Drupal crash - What can I do !!!

As the solution depend on your error, first of all, you must find why drupal crash.

1. First thing is, You must check the serveur error log.
# tail -f /var/log/apache2/error.log

2. Active Error log on your setting.php
error_reporting(E_ALL);
ini_set('display_errors', TRUE);
ini_set('display_startup_errors', TRUE);
$conf['error_level'] = 2;

Some common Errors

Error 500 internal server error.
The reason may be varied, the most common problem is .htacces file.
Change every .htacces files contains Options +FollowSymLinks, replace with Options +SymLinksIfOwnerMatch .

The provided host name is not valid for this server.

Add your domain to the settings.php file, Like:
$settings['trusted_host_patterns'] = array(
  '^newdomain.com\.loc$',
  '^www\.newdomain.com\.loc$',
);

Drupal 8 not Display or Rebuild correctly
And the folder files/php/twig not present

chown DRUPALUSER:APACHE_GROUP -R files/* (On debian/Ubuntu : chown DRUPAL:www-data -R files/*)
chmod 0770 -R files/*

Fatal error: Maximum function nesting level of '100' reached, aborting! in /var/www/html/.../vendor/twig/twig/lib/Twig/Node.php
This is due to Xdebug
Solution : Set xdebug.max_nesting_level on php.ini
xdebug.max_nesting_level=500

MySQL / Database Error

SQLSTATE[HY000]: General error: 1030 Got error 28 from storage engine

Error 28 means database server is out of disk space.
Solutions:
1. Raise the database disk quota.
2. Remove unwanted tables, data ....
3. Clear cache
4. Clear / Remove unwanted contents.
5. Or any other solution so make database space.

NOTE : This error may cause by the mysql server error (error on information_schema db), I don't know the correct way to resolve, but I resolve it by uninstalling and reinstalling the mysql server.