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Sorry to hear you’re facing problems 
help.nextcloud.com is for home/non-enterprise users. If you’re running a business, paid support can be accessed via portal.nextcloud.com where we can ensure your business keeps running smoothly.
In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 18.0.2): 19.0.1.1
Operating system and version (eg, Ubuntu 20.04): Debian 10
Apache or nginx version (eg, Apache 2.4.25): Nginx
PHP version (eg, 7.1):
The issue you are facing: I opened up my nextcloud server today and find that the server has crashed.
Is this the first time you’ve seen this error? (Y/N): N, I have encountered it once, and solved via reinstallation, but this time the files and configuration are valuable, so I want to recover it.
Steps to replicate it: I don’t know the cause, the server worked just fine and now it crashed
The output of your Nextcloud log in Admin > Logging:
{"reqId":"rZjwvAgrkEEjvWGa62tG","level":3,"time":"2020-09-01T07:34:56+00:00","remoteAddr":"60.194.241.132","user":"--","app":"core","method":"GET","url":"/remote.php/dav/avatars/Tangdi/128.png","message":{"Exception":"Doctrine\\DBAL\\DBALException","Message":"Failed to connect to the database: An exception occurred in driver: SQLSTATE[HY000] [2002] Connection refused","Code":0,"Trace":[{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Connection.php","line":889,"function":"connect","class":"OC\\DB\\Connection","type":"->","args":[]},{"file":"/var/www/html/lib/private/DB/Connection.php","line":194,"function":"executeQuery","class":"Doctrine\\DBAL\\Connection","type":"->","args":["SELECT * FROM `oc_appconfig`",[],[],null]},{"file":"/var/www/html/3rdparty/doctrine/dbal/lib/Doctrine/DBAL/Query/QueryBuilder.php","line":200,"function":"executeQuery","class":"OC\\DB\\Connection","type":"->","args":["SELECT * FROM `oc_appconfig`",[],[]]},{"file":"/var/www/html/lib/private/DB/QueryBuilder/QueryBuilder.php","line":216,"function":"execute","class":"Doctrine\\DBAL\\Query\\QueryBuilder","type":"->","args":[]},{"file":"/var/www/html/lib/private/AppConfig.php","line":331,"function":"execute","class":"OC\\DB\\QueryBuilder\\QueryBuilder","type":"->","args":[]},{"file":"/var/www/html/lib/private/AppConfig.php","line":109,"function":"loadConfigValues","class":"OC\\AppConfig","type":"->","args":[]},{"file":"/var/www/html/lib/private/AppConfig.php","line":287,"function":"getApps","class":"OC\\AppConfig","type":"->","args":[]},{"file":"/var/www/html/lib/private/legacy/OC_App.php","line":883,"function":"getValues","class":"OC\\AppConfig","type":"->","args":[false,"installed_version"]},{"file":"/var/www/html/lib/private/TemplateLayout.php","line":164,"function":"getAppVersions","class":"OC_App","type":"::","args":[]},{"file":"/var/www/html/lib/private/legacy/OC_Template.php","line":185,"function":"__construct","class":"OC\\TemplateLayout","type":"->","args":["error",""]},{"file":"/var/www/html/lib/private/Template/Base.php","line":132,"function":"fetchPage","class":"OC_Template","type":"->","args":[]},{"file":"/var/www/html/lib/private/legacy/OC_Template.php","line":334,"function":"printPage","class":"OC\\Template\\Base","type":"->","args":[]},{"file":"/var/www/html/remote.php","line":87,"function":"printExceptionErrorPage","class":"OC_Template","type":"::","args":[{"__class__":"Doctrine\\DBAL\\DBALException"},500]},{"file":"/var/www/html/remote.php","line":169,"function":"handleException","args":[{"__class__":"Doctrine\\DBAL\\DBALException"}]}],"File":"/var/www/html/lib/private/DB/Connection.php","Line":67,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Windows) mirall/2.6.5stable-Win64 (build 20200710) (Nextcloud)","version":"19.0.1.1"}
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'memcache.local' => '\\OC\\Memcache\\APCu',
'apps_paths' =>
array (
0 =>
array (
'path' => '/var/www/html/apps',
'url' => '/apps',
'writable' => false,
),
1 =>
array (
'path' => '/var/www/html/custom_apps',
'url' => '/custom_apps',
'writable' => true,
),
),
'instanceid' => 'xxx',
'passwordsalt' => xxxx
'secret' => xxxx
'trusted_domains' =>
array (
0 => 'cloud.ixxxxxcx.ink',
),
'datadirectory' => '/var/www/html/data',
'dbtype' => 'mysql',
'version' => '19.0.1.1',
'overwrite.cli.url' => 'https://cloud.ixiexie.ink',
'dbname' => 'nextcloud',
'dbhost' => 'mariadb',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'nextcloud',
'dbpassword' => 'xxxx',
'installed' => true,
'maintenance' => false,
);