Zoas and palys are weird creatures, and can act that way for several different reasons, and sometimes for no reason at all. First thing people are going to tell you is check all water perameters and check for pests and diseases. If there is no obvious reason, start looking into other factors: how much light/flow does it get, what is near it, how long have you had it. Sometimes zoas can close for a while to do the natural cleaning process, or when it has caught some food and is digesting. Is the mat still a good brownish to purple, or has it became dark or blotchy? That can be signs of pests or disease and is followed by melting. Several coral dips on the market can help, but I'm not an expert on those, and from my experience with zoas, sometimes there is just nothing you can do.
Try relocating it and inspecting it thoroughly, and post pics please.