Is it possible the root password's expired? If the root account password has expired, you won't be able to log in as root from another user (su). You'll get an immediate response from the correct password and a delay from an invalid one. Your only option is to log in as root from the console.

You might consider installing a script that notifies you when account passwords are due to expire and you should install sudo. Then your account can have root priv's and you can reset root's password.

Carl