Well, as someone who is not the biggest fan of valentines and has not been married nearly as long, I would suggest looking back. I can't say all or even a majority of women fall into ths category, but when I've heard the "romance is gone" phrase, it usually means she isn't feeling pursued anymore.
Maybe look back into how you won her over in the first place. Instead of a card try hand writing a letter. (Espically if it's not your typical style) My guess is she is looking for you to exhibit thought and consideration. I know, it's a stupid emotionally driven idea, but it's how women are wired. We know we are loved, but we want to know you are "in" love. You know her better than anyone else on the planet, do something instead of buying something. And good luck!
"Indeed, this seems actually to be a psychological and sociological law: the power of some needs the folly of others." -Bonhoeffer