|I would advise taking it to your company's IT dept. They will know how to re-connect it to the domain safley without any major issues.|However that being said, if you still want to go ahead yourself.
Right click on "My computer" and go to "Properties". Go to the "Computer name" tab and then click on the button "Change".
In the dialogue box you have the Computer name, and two selections (Member of domain and workgroup). Select Domain and enter your company's domain name.
Windows will then try to re-join the domain.
EDIT: Just re-read and noticed you cannot get in. You will either need the office to supply the local "Administrator" password on the computer, or get them to re-join it to the domian.
message edited by thomasad