Help

Ok, I have a problem and I don't know what I've done...let me try and explain. I'm using Photoshop 7 and usually when I take pictures/screencaps and I erase everything I dont want in the picture and recopy it into another document it will lay over the background I've made before it with ajusting anything. Just now when I went to do it, it didn't 'overlay' (for lack of a better term) and its just all white where I've erased what have I done wrong?