This might sound like a silly question, Juliano, but since the re-boot has anyone logged into the domain?
The way MLC was described to me is that it reads the login events from the security event log on the domain controller. If everyone is already logged in, and no one has logged in since the re-boot, there will be no new security events for the login collector to see.
every day, the employees log in the domain when they come to work. So, everyday we should have new login events. But, oddly, I just get the message saying that there is no loggin events. I think that it may be happenning because of some windows update installed after the reboot.