...Sounds like the death curl. Just sitting on its back, waving its swimmerets and not moving at all, except the occasional spasm or dart around the tank, ending in the same position in a different place.
Definitely check your water quality. How long did you acclimate? Did you temp acclimate before you put it in your tank? This is a pretty new mantis, right? This happens most commonly after acclimation, maybe a couple of days after. It happens because of stress such as improper acclimation, bad water quality, or just plain stress. Also happens from bad molts. Seems most common in wennerae. Most you can do is maintain water quality and turn out the lights. Don't go near the tank more than a couple times a day if you can. Also someone once said that an airstone in the tank will help, though i can't confirm or deny that.
Dan