While you are in a nonpay status, you will be able to prepay your account by accelerating your allotments prior to your period of nonpay or you can freeze your account. Activation to military service is considered a qualifying life event, so you have the additional option of canceling your election for the remainder of the year and reducing your coverage to the amount deposited or what you have already been reimbursed as of the start of your leave.
The Federal FSA Program Quick Reference Guide for Leave Without Pay provides additional information about your options while in a nonpay status.
If you are a Qualified Reservist, you may be eligible to request a Qualified Reservist Distribution under the provisions of the Heart Act in order to avoid forfeiting money in your FSAFEDS account.