everytime i leave my computer for about 10-15 mins, the screen goes black(normal), and then when i go to use it, i move the mouse and the desktop appears(normal), now the problem is..now when i leave the computer for about 15 mins it goes black but when i move the mouse the log in screen shows up and i have to click on my user name and then it goes to the desktop? how do i make it like before?