My lg is 20 months and she has started waking about 1-2 hours after I put her down for the night. She cries, not frantically, but every 30 seconds or so, which escalates until she is crying constantly. I then go to her but end up having to bring her back down stairs with me. She then starts playing and is as happy as Larry. It ends up with me having to go to bed and let her sleep with me otherwise she can be awake and happy until 11pm. Even when I put her in my bed with me she messes about for ages. She has one nap for 1-1.5hrs usually around 11am (sometimes it's hard to get to 11am since she crashes badly in the morning). She goes to bed between 6-7pm. She always lets me know when she wants to go and so I take her up and 99% of the time she goes to sleep without a problem..... Until the crying starts. It's happening more and more often and I don't know what to do.
Could you lay there with her until she starts waking up/getting restless and before she really starts crying to comfort her before she wakes up fully. I suspect her body is treating it as a nap and she actually can't fall asleep again until she's been awake for awhile. If you can catch her before she fully wakes up and starts crying, you might be able to break the cycle.
Do you know why she's crying? Is she having nightmares? Is she scared of waking up alone? Has she developed a fear of the dark? If you can figure out why she's waking up crying, you can maybe do something to help her.
You could also try moving her nap-time later and put her to bed a bit later.