It happens every time you take a photograph with the subjects in front of a window. There is too much light coming through the window, everything in front is too dark and everything on the outside of the window is overexposed. Even if you use “auto mode” on your DSLR, this will happen.
This is where a fill flash is needed. There needs to be enough light in front of the subject of your photo. The fill flash is a supplement to the light already involved in the scene. It isn’t the primary light source; some newer point and shoot cameras actually have a “fill flash” option on them which will add just enough flash to counter the backlighting but not too much that you wash the subjects out. When you are pushing the little “lightning” button on the camera, it will cycle through options: “on”, “off”, “red eye”, and if you have the option, “fill flash”.
More advanced digital cameras will actually have options where you can experiment with the strength of the flash. The best way to figure out how much “flash” to use is to experiment: take a few pictures with different power levels until you find the one that works.