I believe that should be mysql -u root -p xxxx
When you do that, your prompt should change to a mysql> prompt that tells you are actually in. Type status at that prompt to confirm all is ok
-U is not the same as -u , but perhaps you typed the post wrong.
Can you open a database or browse a table from the command line? That will also tell you if mysql is working ok.
I'd also recommend giving mysqladmin a go to tell you more about what is going on.
By default, there is no password for root in mysql, so unless you've set it try logging in at the phpMyAdmin login screen with root user and blank for password.
info on setting mysql root password