I cann't login in backend anymore.

[AloisFellinger]

I am running Magento 1.x in the Austrian version and yesterday, all of the sudden, after generating a delivery note, I was thrown out and since then, I am no longer able to re-enter the backend/admin section. The frontend is still displayed, but no orders can be placed.

Please help!

Thx

Al

[JLHC]

Do you see any error messages in your logs? 

One simple cause for issues like this may be the lack of Disk Space. Are you hitting your Disk Space limit? 

[AloisFellinger]

Disk Space should not be an issue,

The log file entry around the time when this error occurred reads as follows:

a:4:{i:0;s:0:"";i:1;s:561:"#0 /home/www/web786/html/MagentoAT/app/code/core/Mage/Core/Model/App.php(842): Mage_Core_Model_App->throwStoreException()
#1 /home/www/web786/html/MagentoAT/app/code/core/Mage/Core/Model/App.php(491): Mage_Core_Model_App->getStore()
#2 /home/www/web786/html/MagentoAT/app/code/core/Mage/Core/Model/App.php(274): Mage_Core_Model_App->_initCurrentStore('default', 'store')
#3 /home/www/web786/html/MagentoAT/app/Mage.php(616): Mage_Core_Model_App->init('default', 'store', Array)
#4 /home/www/web786/html/MagentoAT/install.php(128): Mage::app('default')
#5 {main}";s:3:"url";s:18:"/install/index.php";s:11:"script_name";s:12:"/install.php";}

[TutorialMagento · Accepted solution]

Turned out sessions couldn't get written to var/session, even though the directory itself is set to 0777. Magento created session files, but they all remained zero bytes.

You can also try to repair your Database.