Enable errors and logging with drush or with settings.php

You have error, your site is not accessable and you need to see what is the problem but logging and errors are set to none.
Type this with drush

drush vset error_level x

Where x is 0/1/2, depending on what you want

0: none
1: errors and warnings
2: all

You can also do the same with settings.php, adding to it this

$conf['error_level'] = 2;