Primary Domain Controller disapear (NT4)

There are two domains in my network that just disapered. After putting these domains in Computer Browser, I could see the domains, but when I enter in the domain it is not possible to find any machine. I ran a Resource Kit application called “DomainMonitor”. In these two domain the message was “could not find any PDC”. What are the troubleshoots that I can make solve this problem? Thanks in advance.