If the DC is offline how and which domain are you trying to Join???
If you want to unjoin, you could use David's technique. Press CTRL+ALT+DEL twice at login screen and login to the workstation locally with an administrator account, dont login to domain. Then you take properties of My Computer, go to Network tab and make your workstation member of a workgroup.
Let me know how it works for you...
Another way is, issue this command
C:>netsh interface ip reset reset.log
Your TCP/IP stack will be reset, all errors gone all memberships gone.