Camera Questions

My beloved Nikon D40 took wonderful pictures at camp. Then, all of a sudden, the pictures became darker and darker. You can barely make out the images in the photographs. I have tried to push the reset button. I am sure I have caused even more harm by trying to fix it. I need your help! The pictures are dark but my flash works. I have no idea how to fix it. I have a new memory card, tried the old one again, and have charged the battery. For a while I had a "shutter release error" message, but that is gone now. What can I do to fix my camera?

And, if it has bit the dust... should I buy another Nikon or try the Canon Rebel? Help!