I was doing the quest where Serena and I enter the Soul Cairn and I put all the stuff in, but now she won't give her blood and follows me everywhere. This is preventing me from doing any other quest and can't sneak because she is around. How do I fix this?
The quest is Chasing Echoes and you are trying to open the Soul Cairn, correct? The Elder Scrolls Wikia page lists a few known bugs with suggestions for fixing them.
On PC, you would find this bug relevant:
After all the ingredients have been gathered, Serana stops moving and thus she will not cut her arms and the quest will not progress, to fix this, type setstage dlc1vq04 90 in the console after speaking with Serana.
On xbox/playstation, you would want to refer to this bug:
Sometimes when placing the items in the Portal Vessel nothing will happen other than the loss of the items. The quest will not be able to be continued if this happens and an earlier save will have to be loaded to try again.
The UESP page for the quest lists similar bugs and has additional suggestions for fixing them on the PC. Keep in mind that on console, you will most likely have to load an earlier save.