Domain is not available

I am having a major problem.

I have 2 DC. One PDC and one BDC. I have Active Directory replicating to the BDC from the PDC (before I installed the BDC the PDC was the only DC). All was running well until the other day the BDC crashed. Now when people logon the get the message “The system cannot log you on now because the domain is not available”. When people try and browse the network the get the message “There are currently no logon servers available to service the logon request”. I have checked DNS and it seems to be working fine. I can ping the DNS server without a problem. It seem like all the computers were logging on to the BDC. Now that the BDC is down no one can log on, even with the PDC having all the same info.