Multisite restore problem

My sister had wordpress installed in your domain and had some errors during update from version 3.5 to 3.5.1. She couldn’t login and no one of sites could be accessed.

I backed up wordpress folder and database and tried to fix restoring wordpress instalation, now I can access the main site but I cannot restore sites, during the process I get this message:

Warning: An existing WordPress network was detected.

If I continue the process of installation I get the same inital problem, no sites can be accessed and I can’t login anymore.

How can I restore the multisite database in a clean installation of wordpress?