ubuntu mate - No password accepted after restarting GUI

I was trying learning Linux I was doing the exercise of turning off and on the GUI. But the command on the teaching course did not work. So I used the recommended by AskUbuntu

How do I stop GUI?

So I used the following command

sudo service lightdm stop

then I used the same command but with Start

 sudo service lightdm start

And now... I get a new green display that says Ubuntu MATE and it does not accept my password.

Can somebody tell me what when wrong? and how to fix it.