If it makes you feel any better, the thing about art block is that if you have it, you are starting to see the flaws in a piece, therefore you are improving. I recommend doing anatomy, landscape and still life sketches. It is boring but it really helps if you practice at least 30 minutes to 1 hour a day. At least that is what I do when I am stuck. It seems to work for me so far.
This may sound weird but I like the look of that artwork. I guess I am just a sucker for supernatural fantasy looking stuff.
If I may make a suggestion, the shading with the city she is about to stomp on and the beginning parts under her foot up front would look better with shades of blue. I do like that you kept values in mind, though. You know, with the sky being above meaning that the shading will be in the bottom because there isn't as much light source under there.