Admin user missing !

Hi,

My website was hacked yesterday (defaced). I don’t know if it has something to do with that, but, even if I restored an one-week-old backup, I have a strange problem :

The “admin” user disappeared !

In the “Users” page in WordPress: all users appear, except “admin”. Over the list, it appears “Administrators (2)” (admin + another), but when I click on it, only 1 appears (the other one).

Then I went to phpMyAdmin, and in the table wp_users, “admin” is still missing: it starts with ID “2”. But in wp_usermeta, “admin” appears correctly.

And when I try to connect with “admin”, it tells me “wrong login/password”.

I don’t know what to do to correct that…
Please give me some lightup…