Change in logon after change in network setup

After I’ve got my old company computer for my private use, I’ve changed a setup under My Computer from Being a part of network to Using computer privately and now I can not log on! Before I had my username and my password and a domain name and everything functioned well. Now after I have changed from Networking to Private use there is no Domain in the Logon window and my old password that was created for the old company network is not accepted any longer. What shall I do?! Where shall I get a new username and password? Or can I get the old settings back in some way? I’ve tried this one with F8 – didn’t help. I’ve tried with F12 – didn’t help either. Must say I’m an amateur – don’t even know what “booting” means :o(((