I would'nt say permanent, but a slow death can begin along with infections. The zooxanthellae that live in the corals will begin to die from lack of light & they are partly responsible for feeding the coral. They also play a large role in providing those beautiful colors that we love. So after a week or 2, it is likely that the coral will begin to lose color & eventually start dying. Also, bacterial infections have been known to set in from long periods with no light. Iodine solutions usually rid you of that problem, but stress the coral somewhat. I hope you've been dosing with iodine atleast once a week. Iodine keeps everything healthy & a healthy headstart is what you need in this hobby. Good Luck