Screen shuts off/sometimes wont power on

So my girlfriend has been having trouble with her Macbook and I replaced the magsafe port a few weeks ago and so far so good. Now when you set the computer down on a hard surface (table etc) the screen will turn off and sometimes will just shut off. I believe that it is going to sleep when this happens but it will be unresponsive when I try to restart. I can hear the computer starting up and running but no display. Also the fans only run some of the time. Any help?