I wonder if someone has had this same problem and can give me some advice...
DD (20 months) has had a couple of episodes in the last week where she wanted to sleep but couldn't. She has always loved bed time, so that's not the issue, it is not a struggle to take her to her cot at all, she loves it, she hasn't got any bad associations with bedtime or anything like that. But there have been a couple of nights where she has waken up and then she is unable to go to sleep again despite trying and trying (she doesn't stand or doesn't play, she just lays down trying to sleep). She talks a bit already and she says she wants to sleep, and she lays there in her cot, but moves a lot because she cannot get to sleep. The first time was horrendous, she woke up about midnight and she didn't get back to sleep until 5am! I tried everything (normally just holding her hand will do for her to go back to sleep), but that day I took her in my arms, put her on top of me in the sofa, tried the pushchair...when I took her out of the cot she wanted to go back because she WANTED to sleep but she just couldn't, it took her 5 hours that first time, it was horrible, she was crying in frustration at the end 
She had the same problem tonight, not as bad as the first time, as she went back to sleep at 2am, after 4 hours since she woke up...it was not as bad as at least she got some sleep and was not crying.
I am worried as it has happened twice in a week and is something completely new to me. I hope this is not the beginning of a pattern...?
Has anyone any experiences about this? Are there any triggers for this? Everything I found online about insomnia is about children who do not want to sleep, but that is not my DD's case, she WANTS to sleep and tries but cannot do it.