Cannot Authenticate Users from Trusted Domain

Hi.

My organization uses SharePoint 2010 (version 14.0.6109.5002).

We have a group of users from another domain that used to be able to access our SharePoint site.

This domain has a Full Two Way trust with our domain.

These group of users have recently been unable to access our SharePoint site.

The interesting part is that we are no longer able to resolve the account from this domain when we try adding them from the permission section.

What could have gone wrong?

Our AD replication is working fine. Is there any area that I should be looking at to resolve this issue?

Appreciate any help I can get.

Regards,

Adrian

Adrian