No image after layer merge

[Mithral Londrecken]

I  to a photo of a foot print made with graphite powder. then in gimp i cropped it to just the foot print. Then I used select color to copy the pixs to layers. I have spent hours hand drawing to remove the pixls grainyness for each layer, making depth map of the foot.  Now when I merge the layers my image outcome is blank.  What am I missing?? have not found help on this in youtube. just want to git this inage through to the 3d process but stuck.