My character did not go to Soul Cairn as a vampire. Her soul was trapped by Serana into a soul gem. Was able to get the Blood scroll. But on the way back she was not able to retrieve her soul essence gem. Returned to Skyrim without the soul essence, stats seemed normal, what will happen to her now that she wasn't able to get her soul back? Do I have to go back to the Cairn to find it, btw I was not able to get the quest too even after talking to Valeria about it. Pretty confused now. Thanks for your help!

I usually find it myself. After all, there's a lot to look for out there: Arvak (the horse which doesn't die because it's already dead), 3 new spell tomes, Saint Jiub and ofcourse, the Reaper

As for what will happen if you don't get it back... I believe the weakness effect is strictly Soul Cairn related. This, means that your character will be weakened each time you set foot in there -till you find and get it back ofcourse.

If you did not get the quest to get it back, and you got all the plunder in the soul cairn, then you got the gem back and did not realize it. I had that happen to me a couple of times. One of the masters who guards a chests has it, and you must of found the chest that had it. If not, you would have gotten the quest to get it back. Seeing as you did not, then you probably already found it. If you did not find it, she would have given you the quest to get it back, and it would also have a quest marker. Hope that made sense.

Hope that helps