You can't get it back unless you put in a claim for expenses incurred during the time of employment. BUT (from my HR Director wife):

1. You can put in a claim for the entire amount elected. So, if you elected $1200 for the year ($100 per month) and left in January, you can put in a claim for $1200 even though you only contributed $100 (or perhaps less).

2. You can COBRA it. So, in the first example, you can elect COBRA for February, pay $102 (the 2% administrative fee) and spend $1200 in Feb.