My hermit crab came out of his shell and won't go back in.?

I've had my hermit crab for 4-5 months now. He has ALWAYS moved back and forth between shells. I always put his water dish in a corner of his age because he seemed to like to crawl in between the wal and water dish and sit there, with it's antenna's/feeler's in the water. He did this regulary and had no problem getting out. Although today when I went to check on him, he was out of his shell and in the little space between the wall and water dish. His shell was still in position of "lounging" by the pool. I thought he was dead, but when I went to move the dish, he moved a claw. I didn't want to disturb him and I have NO CLUE what I am doing. My mother mentioned him molting? But I said it was probably unlikely due to the fact he was not under ground and he was out of his shell. I would also like to mention that the crab is greyish in color, is this normal? I am a first time crab owner... Please help! I love my crab soooo much!