What's the Issue?
A lot of new users are thrown off by an inconsistency in the Ubuntu interface. When you enter a password in
the GUI (graphical user interface, where you point and click), there
is visual feedback—a dot or an asterisk for every character you type.
When you enter your password in the terminal, the terminal doesn't
seem to "accept" your password because it doesn't offer any visual
feedback for the characters you enter.
How do I solve the "problem"?
Your password is being accepted in the terminal. You just aren't getting any visual feedback. Go ahead
and type your password and hit Enter afterwards. If you typed your
password correctly, there should be no problems.
Though I agree with the following comment from bodhi.zazen
It is not "an inconsistency in the Ubuntu interface", as far as I
know, this is the default behavior of both sudo and su on all distros.
It is unfamiliar to new users perhaps, but not an inconsistency.