You won't be able to change the domain admin password in DS restore mode
since Active Directory will not be loaded. However, in restore mode you
will be able to restore from a system state backup (which will have the old
password).
Can you provide the exact error message you are getting when you try to log
on? If the username or password are wrong, it will be "The system could not
log you on. Make sure your User name and domain are correct, then type your
password again." If you are getting a different error, then it is not a
password issue.
Did you make any other changes when you changed the password (security
policy, user rights, etc.)?
You might also try different variations of the new password (all caps, all
lowercase, misspellings, etc) in case it was typed wrong when it was
changed.
It is also possible that someone else changed the password after you changed
it so you may want to look into that as well.
Once you recover from this, I would recommend having at least one other
administrator level account to prevent problems like this in the future. It
is also best practice to rename the administrator account to help avoid
password attacks against it.
