reason of the downtime was the HDD running out of space, so mysql was not able to create temporary tables anymore. a clean shutdown failed because the .lock file was still there so I had to kill -9 mysql.....which corrupted some open tables.
usually no big deal because you can repair tables marked as crashed with mysqlcheck, but it failed this time, so I tried myisamchk which was able to restore the indexes of the 2 tables marked as corrupted. afterwards the site loaded fine but all content was missing....the posts showed up in the database in list view, but when trying to view the content it returned an empty result.
so I repaired the wp_posts table (it was a Wordpress site), which has a size of almost 4 GB, with myisamchk too, even if it wasn't marked as corrupted or crashed....which worked!
