My 22month Old ALWAYS Cries in the night

I have been putting my daughter down for naps and bedtime alone since she was 6 months old, however she has gone from being  excellent with this routine to crying every time. She will sometimes cry and moan for a couple of minutes, but lately she's been crying upwards of 20 minutes. In the middle of the night she will wake and cry on and off for almost 2 hours if not more. I was worried and got her a few times, however she wasn't having nightmares and simply wanted to play. Then after staying up with her for an hour or so I would put her down and she would start screaming again for another hour. I am absolutely exhausted with this as even though I don't get her in the night I am still up listening to her making sure she is okay. I have a son whom is also kept up by her screaming and I just don't know what to do as I can't get her at 1am or 3am and play with her for a couple of hours only to have her scream for another hour when I put her down. I don't understand why she was once so good at bedtime/nap time and now she seems to have gone backwards. Please help,  I really am at my wits end and don't know what to do???