M.E.
I totally agree with the other Moms who recommended not letting her back downstairs as this will make the issue worse. Our son is 27 months and has been a very good sleeper since he was 8 weeks. I have to say that we find ourselves "reinventing" his bedtime every couple of months, while keeping it quite routine. Sometimes he cries and he almost always he asks for one more story or one more song - with tears in his eyes. As hard as it is, kissing him, telling him that we love him and closing the door (with his lullabies & nightlight on) is what works best for us. Just remember that your daughter needs to know that she is safe and loved more than anything, so it may take time in her room to get her back on track - but don't give in and let her out of the room once your nightime routine has begun. This too shall pass!!! Good luck!
One other thing that my sisters with grown children keep reminding me of ... the second year of life is all about asserting one's self. Remembering that tends to make each little battle that much easier to hurdle!