Well you know this is the second time I have heard of something like this recently. Of course you can always write a letter and ask to sail again at a later date for what you paid, but unfortunately the cruise line is under no obligation to do so. Did you by chance purchase travel insurance? One would think that if you were denied boarding due to a health related issue Travel insurance would reimburse your expenses. If you did buy trip insurance, most of the companies want to be informed as soon as you have incurred a loss.
I am sorry your trip was ruined, and hope you can find a way to salvage the money you spent. But if nothing else, it was nice that you took the time to post so people can see the value of trip insurance, and that it is a worthwhile investment that can help you recoup what would otherwise be lost.
I hope you will prevail!