Bring up Computer Management on your machine then try to connect to the Exchange server so
1. Right click My Computer
2. Click Manage
3. Right Click Computer Management
4. Click Connect to another computer
5. type name of Exchange Server eg. usaex01
6. Browse to Local Users and Groups
If it says Access Denied, then someone has messed with the security on the server, if it shows you, then browse through the groups and check the administrators etc poke around.
My guess is, someone has altered the local security. Have you tried logging in with different accounts?
Whats the actual error you are getting ? We need a bit more information as it could be 100 things :)