Installation of WSE2012 Connector failed and stuck on lockscreen

Installation of WSE2012 Connector failed and stuck on lockscreen

I have set up a PC on my network with Windows Server Essentials 2012. I
created several users and I wanted to connect my PCs with these users and
my server.
I install with success the Windows Server Essentials 2012 Connector on a
Win 8 Pro. I tried to install the Connector on a Win 7 Pro. Everything
worked well and the PC reboot.
After the reboot, an error occured (I think a network problem, I don't
remember exactly -_-). It canceled the installation of the connector.
Now, when my Win7 boot, I am stuck on the lockscreen where I am asked to
enter login/pwd. If I enter the login/pwd of the new AD account, it failed
(neither with the admin account nor the standard user).
I also tried to connect with the old local accounts but it failed to. May
be the problem is beacause the local accounts don't have a pwd ?
The only solution I see now is formating the Win7 PC. But I'm still hoping
someone here will find a solution ...
Thanks in advance and sorry for my english, I am not a native speaker.