Use the "-p" option and just press <enter> when asked for a password, if you have not set a password:
mysqladmin --no-defaults -u root -p version
If you have set a password but you've forgotten it, then you need to stop mysqld and then restart it, using:
mysqld_safe --skip-grant-tables -u root
You can then set a new password. I suggest you to read this
to see how to secure your installation of mysql.