I feel for you! We've been through this recently with our 26mo old daughter! It could be a few things, it's a lot of trial and error, since they can't totally express themselves or understand their feelings yet. Sorry, this is gonna be a long post, but these are all the things we've tried. Hope it helps!
The first thing we did was explained the routine; we always talked about the routine before we started it, to give our daughter a "head's up." For example, "Ok, it's nighttime. First, we're gonna go upstairs, put your jammies on, brush your teeth, then we'll go in your room and read 2 stories. And then you and George will sleep in your bed (Curious George, her favorite stuffed animal), and Mommy & Daddy will sleep in our bed, and we'll see you in the morning. Explaining the routine every day for the first couple weeks seemed to help, because she always knew what was happening next.
Does your daughter have a lovey or favorite animal that she could cuddle with at night? Our daughter loves her 'nighttime George" because his face lights up and it plays a lullaby, and it makes her feel like she's not alone in her bed.
Also, we put a nightlight in her room so it's not totally dark. We close her door just leaving it open a crack, and have a gate on her door so she can't get out of the room. That's been a life-saver for us, because it's a shorter trip back to her bed when she does get out. When she does wake up, we just put her back in bed, tell her goodnight and we love her and leave. A few nights this was tough, we'd do this like 6 times. But eventually she figured it out.
Something to consider is if she's developed some sort of fear of her room, the dark, etc., since it's normal for toddlers to start developing fears at this age. Our daughter had a brief fearful stage, after we were out of routine for a couple weeks (my husband was gone for work for a week, and then I was gone a couple days for work, all within a 2wk span.) I couldn't figure it out initially, we asked her if she was scared of something in her room and she said no, but she looked so fearful when we said goodnight. But then I realized she was afraid to go to sleep because she didn't think Mommy & Daddy would both be there in the morning when she woke up. It was similar to when we started taking her to "school" and she was so afraid we wouldn't come back at first. The change in routine threw her off, and so we had to remind her of the routine again, and then we'd tell her that Mommy & Daddy would both see her in the morning (when it's true), or if it's a night when one of us may be gone, then we give her a head's up, "and Mommy will see you in the morning, but Daddy will be at work, he'll see you after work tomorrow." The first night I explained it, she still woke up twice, and kept explaining it the same way, and that was it. She's been fine since! (knock on wood!)
Good luck!