When i leave my pc alone for while the screens turns black which is fine, when i return and move my mouse it comes back on but my second screen stays black and my first screen has a blue overlay over the top of the unlock screen.

I believe this is to do with my dual screens on gts450. screen 1 is 1920*1080 screen 2 is 1024*768 the blue over lay is on screen one and looks to be the size of screen 2

I can out the password in and press enter and then everything goes back to normal, but how do i fix this issue of a blue overlay stopping me seeing whats on screen.