cure my cat's mystery illness

Her illness was getting progressively worse, her muscles were mostly gone, she had lost more than half her weight… it was eating her from the inside. Plus she was too weak to walk straight, when she shook her head she would fall over… I wanted her to go out with some dignity left. So the vet gave her a nice shot which let her finally relax, and she looked so peaceful. I almost did not want to do the second shot, but it was necessary. Perhaps the hardest part was putting away all her belongings.

She keeps licking and chewing at nothing and then shaking her head. Then she grinds her teeth a little.
But these are minor symptoms compared to the main one: She doesn’t seem to want to drink water anymore. She goes to the bowl (which I refresh many times a day) and just stares into it. She might touch the water’s surface with her whiskers or chin but doesn’t drink. Because of this I didn’t notice right away and now it seems serious.