It probably just need watered and it will probably pump back up if it is still green. The hardy Optunia lay down and shrivel up when they are without water for a long time. They will be green but shriveled. That is how they survive when they do not get water.
I think that cactus the pads are pretty flat and not as plump as the hardy one. The one I saw at Lowes the leaves are a little thicker than leaves on trees.
I would moisten the soil just enough that it feels moist but not wet. Once they shrivel it takes about 2 weeks to plump back up.
I am not sure about this Optunia because I have not grown that one so I am saying all of above because of what I have done with my hardy cactus. I have left the hardy one sitting on the ground out of the soil all summer it shriveled flat but was green and once I put it in a pot of moist soil it revived itself.
