after you've selected the area you want to copy make sure your on the layer you want to copy then hit ctrl C

when selecting what layers you want to use make sure you do it on the layers window, if you just click on the picture you will loose what you've seletced especially if you have the lasso tool selected.

if you want to get what you've selected back then hit ctrl alt Z to step back (this will only work during a session you cannot step back when you've just opened a recenlty saved document)