How long has it been closed for? Often times a leather coral will shed it's mucous coat, and when this event occurs, the coral closes and the polyps close (or barely open). The time it stays closed can vary from a a day to a couple days to a week or two [or so I have heard]. Just be sure it is getting good moderate flow to aid in the shedding of the mucous. If you're in doubt, and the coral is removable just take it out of the water for a second and give it a whiff to see if it is still alive. HTH
Also, this is the LPS forum [and a leather is a softie], so you may want to post it in the soft coral forum for more feedback.