Primay Domain Trust Issue

I am the help desk technician for a military hospital of about 400 users.I have 2 domain x and y. I have exchange 5.5 as mail server. I have been getting “Trust Relation ship between primary domain and trusted domain failed” error number 0xc00206fc. I just received a call from a user that received the same error as he was attempting to log in. My question what is the root cause of this error and where do i go to correct it.