You can try going back to the first ruin and looking for her body again. Did you find her dead body and the journal wasn′t there?
It might be possible to fix it by advancing the quest through the console, or maybe by adding a copy of her journal to your inventory, also through the console.
Here′s the http://elderscrolls.wikia.com/wiki/Console_Commands_(Skyrim), for reference.
To advance the quest past the journal -
setstage DLC1LD 160
Including her inventory?
Either that is a bug, or you misread the inventory-part. The journal should be inside the death body of Katria (to be found in the name-giving ruin of that quest).
If it is not there, you can either check again with an old save, or just use the console-command from NutBurz.