after usermod can not su to root

[zhang zhengquan <zhang.zhengquan@gmail.com>]

Dear debian users,

I used to use islanddancer as username to login to my sid laptop. and
islanddancer is in sudoers file.
today I changed it to zhengquan using

sudo usermod -l zhengquan islanddancer

I did nothing else.

Now I can login using zhengquan but I was not able to su to root using
the root password.
also 'zhengquan' is not in sudoers file so now I can not edit
/etc/sudoers to include zhengquan.

Any help to save my system is greatly appreciated!

Regards,
-- 
Zhengquan