A.D.
We had a similar situation at about 13 months and it was frustrating! She had been sleeping so nicely, then we were back to middle of the night wakings and I was at a loss!
Here's what I did that worked for us (we still have the occasional rough night, but overall, very good!)
Whenever she wakes up (and I listen for a bit to see if she's complaining or dreaming or actually awake and upset), I go to her room. Usually she's standing in her crib crying. I go in, give her a hug and a kiss without taking her out of her crib, give her the binky that she often throws on the floor in frustration, and help her lie back down, tell her "night night, I love you", and leave the room. Early on, she'd stand up immediately and we'd repeat the whole scene, sometimes up to 10 or more times. Yes, it was tiring and frustrating early on, but I tried my best to stay even keel and not show my frustration. Each night/nap it got to be less and less. Within a week, we were in good shape. I never had to make her "cry it out", she always knew I'd come if she called, but she also learned that she was not getting out of the crib (unless there was something that needed tending...a dirty diaper, crying like she was in pain, feverish, etc.)
When she is sick or getting a tooth, all bets are off...she's in my arms in the rocking chair as many times as she needs.
So now, I'd say we have 90% of the nights that she's sleeping through, and occasionally I have to go in once or twice to help her back to reassure her and help her back to bed.
It's what feels right to me. Good luck, I hope this helps to at least give you an idea what you might try.