How can I get our 40 year old daughter to acknowledge that we are alive?

You probably have some idea what has gone wrong in the relationship. Were you both upset about something important to her, like a same sex relationship, or an inter racial relationship? Are you in some way not what your daughter wants you to be? Does she think that she is not what you want her to be? You may have an idea about what it would take to heal the breach, and you may or may not be willing to do it – an apology for a one-off problem is different from living with something ongoing that matters a lot to you. If you don’t know what the problem is, perhaps it could be worth putting a note in a card asking ‘We miss you. What would it take to heal the breach?’. At the moment, you haven’t got a lot to lose.

No, I cannot think there is any hope here. There is something missing from this story, and we can't know what it is if we do not speak with your daughter. There are, quite honestly, only two real options. Either your daughter is not quite right and "normal" mentally, or there is something that happened to cause this. Really, this is dislike. It is something more (and less) than you would get as a stranger. But whatever it is, I doubt there is any hope for it.
You might try the following. Instead of telling your daughter what YOU need, you might consider sending HER cards on Christmas and Birthday, telling her you think of her and wish you have a better relationship with you, and should she ever wish to contact you that you are very open to listening to what you might have done to cause her to withdraw from you.
Meanwhile, make good relationships, pretend that for all intent and purpose you never had a child. Make good friends, have reciprocal network set up in which you help others to get to appointments, negotiate problems, and where you can lean on them in times of need.
Wishing you the best of luck.

Wow. Not very professional really. Obviously there are 2 sides to every story, and we've only read a little about your side. Most likely you can't make her want to have a relationship, but perhaps if you can find out WHY she is refusing contact, it would be a start.

Since you wrote a letter and she responded, that is a little glimmer of hope. She read the letter. So, perhaps you could send another letter, but make it about HER, not you. Just mentioning your age, she claims you are trying to guilt her. So, nothing about you or hubby in the letter.

Ask her how SHE is coping with this virus (those in the medical field have a very difficult and dangerous job - no clue what part of "health" she's in, but probably more exposure than you get.

Ask her if there's anything you can do to help her.

Ask her if she can try to explain why she feels you are try to guilt her (on some level, mentioning your age and asking for "moral" support is a bit of guilt laying - not excessive by any means, but clearly it bothers her.)

Even if she doesn't reciprocate, continue to send her cards for birthday, holidays, just because cards, without asking for anything or expecting anything. Send your love in these cards. Say that if she ever needs help with anything, you'll be there for her. But nothing in these about either of you. Even if there are some medical concerns, try to leave those out. These are only to reassure her that you aren't asking for anything.

Some people may interpret reaching out, mentioning age and asking for moral support will mean asking for more and more later. She may have witnessed this with a friend or co-worker. Unless/until you can find out from her why she has closed the door, it is really all you can do to assure her that the door isn't locked on your side.