It really depends on what is wrong with it. If it was actually removed from the domain and it discarded the SAM for the domain you've got problems. Depending on how or if you backup the system partition, you might be able to restore it and be in business but I am honestly not sure if that would work. If you don't have a backup, I don't know of anything else that you can do but start over and make sure you have at least one BDC if not two so this doesn't happen again. If something has just gone wrong and it has quit communicating, you should be able to fix the problem once your in and get back to where you started. Also, once your in you may be able to supply some more details from the event log etc. that might help someone come up with a solution for you. You may want to wait until some more people weigh in on this also in case I'm overlooking something. Good luck and sorry I can't give you a better answer.