Recently I had this problem. I did nothing to wordpress and the error just suddenly appeared.
I submitted ticket, the server technical personel told me that all the folders should have permission of 755 dan files should be 644. So I went into and changed the permission as told but the problem was still the same.
I searched the net and no solution found.
At last, I went into Cpanel and did the database repair, and suprisingly everything back to normal.
I think some of the spam comments messed up the database. Or maybe some widgets or pluggins.