Owncloud loading blank page

My owncloud after a server reboot shows blank page and not sure what the cause is. created a log report and it shows

An unhandled exception has been thrown:
Doctrine\DBAL\DBALException: Failed to connect to the database: An exception occurred in driver: SQLSTATE[HY000] [2002] No such file or directory in /var/www/owncloud/lib/private/DB/Connection.php:62
Stack trace:
#0 /var/www/owncloud/lib/composer/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(475): OC\DB\Connection->connect()
#1 /var/www/owncloud/lib/composer/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(437): Doctrine\DBAL\Connection->getDatabasePlatformVersion()
#2 /var/www/owncloud/lib/composer/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(379): Doctrine\DBAL\Connection->detectDatabasePlatform()
#3 /var/www/owncloud/lib/composer/doctrine/dbal/lib/Doctrine/DBAL/Connection.php(844): Doctrine\DBAL\Connection->getDatabasePlatform()
#4 /var/www/owncloud/lib/private/DB/Connection.php(148): Doctrine\DBAL\Connection->setTransactionIsolation()
#5 /var/www/owncloud/lib/composer/doctrine/dbal/lib/Doctrine/DBAL/DriverManager.php(262): OC\DB\Connection->__construct()
#6 /var/www/owncloud/lib/private/DB/ConnectionFactory.php(139): Doctrine\DBAL\DriverManager::getConnection()
#7 /var/www/owncloud/lib/private/Server.php(552): OC\DB\ConnectionFactory->getConnection()
#8 /var/www/owncloud/lib/composer/pimple/pimple/src/Pimple/Container.php(122): OC\Server->OC{closure}()
#9 /var/www/owncloud/lib/private/AppFramework/Utility/SimpleContainer.php(108): Pimple\Container->offsetGet()
#10 /var/www/owncloud/lib/private/ServerContainer.php(86): OC\AppFramework\Utility\SimpleContainer->query()
#11 /var/www/owncloud/lib/private/Server.php(1260): OC\ServerContainer->query()
#12 /var/www/owncloud/lib/private/Server.php(424): OC\Server->getDatabaseConnection()
#13 /var/www/owncloud/lib/composer/pimple/pimple/src/Pimple/Container.php(122): OC\Server->OC{closure}()
#14 /var/www/owncloud/lib/private/AppFramework/Utility/SimpleContainer.php(108): Pimple\Container->offsetGet()
#15 /var/www/owncloud/lib/private/ServerContainer.php(86): OC\AppFramework\Utility\SimpleContainer->query()
#16 /var/www/owncloud/lib/private/Server.php(1190): OC\ServerContainer->query()
#17 /var/www/owncloud/lib/private/Server.php(607): OC\Server->getAppConfig()
#18 /var/www/owncloud/lib/composer/pimple/pimple/src/Pimple/Container.php(122): OC\Server->OC{closure}()
#19 /var/www/owncloud/lib/private/AppFramework/Utility/SimpleContainer.php(108): Pimple\Container->offsetGet()
#20 /var/www/owncloud/lib/private/ServerContainer.php(86): OC\AppFramework\Utility\SimpleContainer->query()
#21 /var/www/owncloud/lib/private/Server.php(1458): OC\ServerContainer->query()
#22 /var/www/owncloud/lib/private/legacy/app.php(344): OC\Server->getAppManager()
#23 /var/www/owncloud/lib/private/legacy/app.php(108): OC_App::getEnabledApps()
#24 /var/www/owncloud/lib/base.php(591): OC_App::loadApps()
#25 /var/www/owncloud/lib/base.php(1060): OC::init()
#26 /var/www/owncloud/console.php(74): require_once(‘/var/www/ownclo…’)
#27 /var/www/owncloud/occ(11): require_once(‘/var/www/ownclo…’)
#28 {main}

My best guess: Your data directory was changed and/or is now inaccessible.

data directory is still in tact as before. what should the permission be on the data directory ? think this is possible what changed ?

Owned and writable by the user that is running the web server, sometimes www-data.