You will forfeit any money that remains in your account. Any excess funds are kept by the employer and can be used to offset the costs of administering the program. The IRS regulations require this, and do not allow employers to return the money to plan participants.
To confirm your plan's final date to incur expenses and the last day to submit a claim, log into your online account. To see your plan's filing dates hover your cursor over the "i" (information) icon next to the plan name.