You could try a hard restart (power start) and see if that helps, which resets the game and clears the cache. The only other thing to do is to find a save before you started the quest. One needs to make several different saves in case things like this happen.
The body appears to not be found in any one location.
In my plays I have found the body usually on the path along near the stream, but my most recent play it was at the foot of the Stairway.
From the UESP:
I seem to not understand something about this quest. Every time I started it I done so by finding her body. Is there some other way to start the quest?
From what I just read,
That is all I know. Otherwise you will have to find the body to continue, which is supposed to be somewhere northwest.