Try to access the DC remotely using the <a href=”http://www.microsoft.com/downloads/details.aspx?FamilyID=7D2F6AD7-656B-4313-A005-4E344E43997D&displaylang=en”>server 2008 administrative tools pack</a>. It has a GPMC tool.
You can also edit Group Policy the old fasion way with Active Directory Users and Computers. Just right click on the domain OU and click on Properties.
If you are not able to access the domain using the administrative tools, you need to restore AD from a recent backup.