Well, there are a few possible explanations.
One is that her fingers are sore/swollen after her hospitalisation and she can't get it on.
Another is that she doesn't always wear her wedding ring, and that she decides whether to put it on or not as part of her branding and/or protocol for any particular duty. I think we've seen people posting other pictures of her without any rings.
A third, bit of a wild card, is that it was nicked off her finger in the hospital, which I guess is unlikely in her case but certainly happened to a relative of mine in a less illustrious hospital than the Royal Posh or wherever Kate was.
And a fourth is that she has dumped William, which obviously is a massive secret, but she's a bit stubborn and/or thick, so she insists on not wearing her rings to proudly announce her single status even at the risk of the massive secret leaking out, and then she allowed herself to be photographed without the ring thinking "OMG this is so risky, of course the massive secret might come out but I love it, gonna neck another couple of WKDs when I get indoors too", and then the photo dropped and she thought "wonder if anyone with notice that I've revealed the massive secret", and then she got the posh trouser catalogue out for something to park her new BBL in.
Personally I'm going for option 1, but I predict that some form of option 4 will have its own thread on here by tomorrow morning.