I can't log in and black screen problems!

Hi, earlier on when I tried to log in one of my two accounts on my iMac, the screen turned black and I could only see and move the mouse pointer (the arrow);

while I could successfully log in my other user (both administrators).

Then I switched it off and on again and I wasn't able to log in the first account: when I put the password and hit 'enter' the screen was back at the login page over and over. (the other account kept working when i tried logging in)

how can I log in again?