Well, this is a bit frustrating, and this is the first time I've seen this in the five or so years I've been playing this game, that I can remember. I just got to the top of Yet Another Tower, and the sigil stone is broken. There is no activation point. No hand icon. Nothing. Anyone else seen this? Anybody find a workaround? (I'm on the 360, so no console commands.)

Did you try saving to a new slot, exiting the game, clearing the cache, reloading the game and try to get the sigil stone again?

Aside from that you can revert to an earlier save prior to going into the gate and try doing it again

Those are your options

Load auto save from when u enter the chamber? If you saved and quit, that one auto save should still be intact.