Search no longer works

FESS has been in use for about 2 years I think and always worked pretty much out of the box. I never got to try the AD integration yet, but as for now it was good as it is.
Unfortunately, it stopped working overnight. Two million documents have been indexed, but whatever I search for, returns no hits. All looks fine in the admin interface as far a I can tell, but I’m not a linux admin so I could use a few hints.
Thank you.

Fess supports a secure search. If an indexed document has permission(or role), a user has the same one to search it. So, it’s better to check permission or role in indexed documents and users.