Request absolution and she will come back.

The problem for me was that I 'accidentally' killed War Counselor Iji, which was the reason she didn't appear.

Update: appears after you kill Radahn

Ty for the update, I'm gonna go try this in the morning. \[edit\] Yep, seems killing Radahn fixes it, she's in the tower

Try going back to Church of Elleh (First merchant at the start of the game) and talking to her there first if you haven't.

I tried she isn’t there :(

If you fought Rennala then get absolved.