group policy stopped working

Since yesterday our server started getting this error message in the event log. Any suggestions on how to fix this thanks

Log Name: System
Source: Microsoft-Windows-GroupPolicy
Date: 2013-07-04 12:20:13 PM
Event ID: 1055
Task Category: None
Level: Error
Keywords:
User: SYSTEM
Computer: ISNA-SERVER.ISNA-CANADA.local
Description:
The processing of Group Policy failed. Windows could not resolve the computer name. This could be caused by one of more of the following:
a) Name Resolution failure on the current domain controller.
b) Active Directory Replication Latency (an account created on another domain controller has not replicated to the current domain controller).
Event Xml:
">http://schemas.microsoft.com/win/2004/08/events/event">;


1055
0
2
0
1
0x8000000000000000

57275


System
ISNA-SERVER.ISNA-CANADA.local



1
1947
0
6531
5
Access is denied.

- System

- Provider

[ Name] Microsoft-Windows-GroupPolicy
[ Guid] {AEA1B4FA-97D1-45F2-A64C-4D69FFFD92C9}

EventID 1055

Version 0

Level 2

Task 0

Opcode 1

Keywords 0x8000000000000000

- TimeCreated

[ SystemTime] 2013-07-04T16:20:13.487669200Z

EventRecordID 57275

- Correlation

[ ActivityID] {3D091078-566A-4191-BA2C-08B243E91256}

- Execution

[ ProcessID] 528
[ ThreadID] 860

Channel System

Computer CONTOSO-SERVER.CONTOSO-CANADA.local

- Security

[ UserID] S-1-5-18

- EventData

SupportInfo1 1
SupportInfo2 1947
ProcessingMode 0
ProcessingTimeInMilliseconds 6531
ErrorCode 5
ErrorDescription Access is denied.