It can't hurt to appeal, in writing, to the board of managers, the co-op management compnay, and the co-op attorney. Taking into consideration your personal situation and current market conditions, you may be able to
convince them to alow a sublet, for a finite time period, perhaps including an option to buy clause in the lease. If that fails, and it might, you may need to put the apartment on the market. All the best to you.