The stars are still there. It's just that their light is little compared to the light that is now streaming to earth from the sun. This light is scattered in a number of ways, and the result is that our eyes are "overloaded" by so much light and cannot detect the faint points of light that are the stars during the day. A somewhat odd example might be seeing someone a couple of hundred yards away using a flashlight at night to look for something on the ground. The same person seen in daylight would look odd, but the point is that you wouldn't be able to see the light from the beam of the flashlight sweeping around in the day like you would at night.