Company does not follow terms of severance package

I was laid off of my job, and offered a severance package of 16 weeks of pay, including full health benefits the entire time. I do have the signed letter.

I waited until the last two weeks of my health benefits, and decided to get new glasses and a dental filling while using my benefits. The dentist and optometrist both tried to confirm my benefits, but were told that my benefits were terminated on the same day I was laid off.

I have talked at length to the HR department, but I am getting the runaround. Now, my benefits officially expire today, but I was not able to use them. Am I eligible for an extension? What is the best way for me to get what I deserve?