kaled - 6:48 pm on Jan 26, 2011 (gmt 0)
Assuming that the domain is registered in the company's name, simply email the registrar and ask what their procedures are for resetting the login. It could be as simple as them looking up your company in the phone book and talking to you.
In the UK, the Companies House website lists companies that have ceased trading. This (or equivalent) information may help you convince the registrar you are legit.