What can we do to remedy the unrecoverable error we encounter for any Drush command? We used Drush on our server without any problems up until now. Actually we didn't do / change anything relating to Drush.

But now, on every Drush command we issue, we only get an error like so:


  Drush command terminated abnormally due to an unrecoverable error.


Same error also occurs for Drush status. Only drush --version shows "Drush version: 6.6.0". 

We installed drush via Composer. And the server is already restarted.

What can we do about this problem, which is a huge problem for us?
Make sure your Drush commands are issued from the appropriate subdirectory of your website (change to the directory where your Drupal site is installed and try again).

Since your question did not specify that, it "might" be the reason why ALL your DRUSH commands are failing ...

Refer to this issue for other possible causes (things you may want to check).

If you recently enabled contributed and/or custom modules, try to temporary disable them to check if the Drush issue gets resolved by doing so.