Maybe something went wrong when the database went down. I don't know if sourcedir is set in a config file or on the database but I think that's empty or a file which should set it is missing.
I hope you made a backup?
Okay - the settings.php file was empty ??
But luckily there was a settings_bak.php from not long ago
Don't know how that happened. Might have been when I used FTP - it usually acts weird for some reason :S
Thanks for the help tho, rvec!