Call back. Be reasonable, but firm, and lead the call.
FWIW, it was never suggested that I needed to fly with them within 2 months. That's BS.
Here's how I approached it:
Hi. I'm calling to report a discrepancy in the miles showing in my FF account. This discrepancy is due to a well-known, well-documented error on your website, which is widely known and has been acknowledged by your company. When I saw the mistake online, I googled "frontier airlines expired miles" and found many reports of the same website error effecting other people.
Since the error was yours, I expect you will reinstate my miles and re-set the expiration date to what you had previously shown every time I visited your website.
Here are the details: Your website had consistently shown that my miles would expire on (a date in the future), and that my account had (N) miles in it. This is what your website showed every time I visited, for many months from (a date last spring) to (a more recent date this past fall). Now, when I log in, is shows zero miles, with the miles expired. That's clearly wrong.
Since this was your error, I expect you will make it right, by reinstating the miles and re-setting the date. Can you do that for me now?
When I ran that by an agent, he just asked for the account info, what miles I was supposed to have, and put me on hold (off and on) for a while. He finally came back and said the miles would be re-instated within 24 hours. They were.
I suggest you try the same approach: here's what happened, here's what I expect you to do, can you do that now please? be polite, but firm, and give them an opportunity to do the right thing. I was prepared to escalate to all sorts of things, but that wasn't necessary.
Good luck.