I can't find this chick anywhere. I'm guessing it's a glitch because I've looked for her using the quest marker on every floor of the castle with no luck. So if I had to guess she's either stuck in a wall somewhere or her hundreds of years alive have allowed her to hone her hide-n-seek skills into a fine point.

Anyone know a way to fix this?

Sometimes she is sleeping in one of the many coffins in castle, but I'm sure you've probably check there too.

I have not had this issue personally, but usually when an NPC disappears from the game (or gets stuck), they're gone for good. You could always try waiting 31 days away from the castle to see if she resets, but this usually doesn't do any good. The best solution would be to reload a save prior to this happening, assuming you have one.