My (29M) partner (30F) says she doesn't feel appreciated, but won't tell me what I could do to change that.

My partner and I have been together for about 4 years. We had an argument this morning, and during this, she told me that she feels unappreciated. When I asked her what I can do to help her feel appreciated, she said that it doesn't matter, because I never "see" what can be done, and that it would be pathetic for her to beg for my appreciation.

I love my partner very much, and I would gladly do just about anything for her, if she were to ask. But how am I supposed to know that something is bothering her, or what is bothering her, if she won't tell me? I am very confused and a bit frustrated because of this situation.

What do I do with this?

TL;DR: My partner says she doesn't feel appreciated, but won't be specific about what I could do better. I don't know how to go about this situation.