if the domain is not visable, then you have probably been removed from it for some reason. If this is the case, you will not be able to log on due to the Account using a cached domain Guid. You will have to log on via a local account to get on the machine.
if you have check the option button on the log on screen and you can see the machine name, not the domain, change that to the domain. Then you can log on.
If you don't have the a local account, you are kind of SOL...
There is a tool that will get you past this, called ERD commander 2k or XP made by Wininternals, that will allow you to boot into the machine (NTFS is assumed) and change the local admin or user passwords without needing to log on. Very usefull for this kind of situation. Problem is the software is $200 for a single machine liscence.
Hope this helps...