How to re-create lost computer account in AD

A computer account was accidentally deleted from AD after the workstation was joined to the domain. Workstation can still log on successfully using the user account. But I cannot log on with the server administrator account because I get the message “domain not available”. I re-created a computer account with the same name in AD, but this is clearly not associated with the workstation.
My need is to un-join the workstation and restore it to workgroup status, but I don’t have an administrator logon to the workstation.