Win2003 can’t change password or add users in active directory

For some strange reason, suddenly I cannot make any changes to users in the active directory of a remote server that has been running several years. EG If I chnage a users password I get:

Windows cannot set the password for because:
Windows cannot find the network path. Verify that the network path is correct and the destination computer is not busy or turned off. If Windows still cannot find the network path, contact your network administrator.

Similar happens if I add a user.

Can anyone PLEASE tell me what is going on????
Thanks.