If you don’t have backups, you should saveas-html-complete all your pages as cached in google asap. You can rebuild your site from those.
Did you save a backup file of your site? (Which is done by going to admin then tools then export)?
If so, you may be able to restore your site by going to admin > tools > import.
Do you have direct access to your MySQL database? Do you have backups of it?
I deactivated then reactivated each plugin one by one. Seems to have been caused by a facebook plugin.
