This is a very sad situation, especially considering your 25+ years of marriage and I presume therefore, a long relashionship with this woman, your step daughter. Perhaps the most hurtful thing for you is to have become the person who is seemingly so despicable, has behaved so dreadfully (though your SD will not state what, exactly it is you're supposed to have done to have caused her reaction to you) that she refuses contact with her dad and cannot bring herself to even acknowledge/thank him for his birthday gift to her.
Now, I'm supposing you have not committed some unforgivable, heinous crime toward her and on the basis of that, I would suggest you do absolutely nothing. If you have done nothing to prompt this treatment of you and your DH then writing to her, paying for her to visit, wringing your hands over what you can do to get through to her, all of this is clearly falling on deaf ears. She is a grown woman with children of her own. If anything were to happen to her dad (and he's not a spring chicken, as it were) then that is NOT for you to deal with. It will be her sadness and only she knows if it will be worth it, this ostracising of her you both. She clearly thinks she has good reason for behaving as she does. You have been very ill and surely you're at an age where frankly, you ought not to be having this. Between you, you brought us 5 kids. They have all been through uni and done well; they're married with families of their own. You get on well with the others and you should be proud of what you've achieved, the two of you.
Let you husband continue his relationship with her. If she won't acknowledge you, unfair as it is, then let her go. There may be a time she wants you in her life again and then, it will be up to you.