I wasn't able to connect to one of the server and after looking into it, it turned out to be DNS issue which is in turn caused by AD replication failure...
coz our whole environment was built upon VMWARE, I noticed that VCB backup can freeze DC and make it fail.
the way to fix this up is to manually restart KDC service. re-enable inbound and outbound replication on the failed DC.
the following command might be helpful:
repadmin /options DC1 -DISA.....