At a loss: At 16 months, she suddenly stopped sleeping!

About a week and a half ago my 16-month-old suddenly stopped sleeping. Almost entirely. She goes down easily at 7:30-7:45, which has been her bedtime for as long as I can remember. Almost exactly two hours later she's up, and will NOT go back to sleep. She'll relax in my arms with her eyes closed, but if I try to put her down she starts squirming and shaking her head no. If I do it anyway she starts wailing. She does this for anywhere between 2-4.5 hours, every night. Previously, if DH would try taking her for a bit she'd scream bloody murder, but last night (after she'd been up for two hours) he took her out into the livingroom and they hung out and played for about an hour, after which she went back to sleep.

After that, she'll sleep for another two hours or so, and then only sleep for half an hour at most if I put her back in her crib (this starts at about 3:30 or so). This forces me to sleep in the chair where I nurse her at night if I want any sleep at all - she'll sleep for about an hour at a time this way, but it's not all that comfortable for me and by the time she's up for the day (at about 6:30-7:00) my body just craves to lie down.

Plus, her one nap during the day now lasts for 30 minutes at most; sometimes I leave her in her crib asleep and then hear her babbling away not two minutes later. Of course, this means that I can't nap either.

She's all smiles and sunshine during the day, so she's obviously not suffering for getting so little sleep. It's only during the night that she's such a nightmare! I don't think it's a matter of my putting her to sleep too early - she falls asleep pretty quickly. Plus, DH and I have gone out in the evening a couple of times since she started doing this and her going to bed a couple of hours later than usual makes no difference; she still follows the same pattern.

Has anyone else dealt with this and found anything that helps? I haven't changed my diet at all, and she's never been sensitive to any foods I've eaten. She just recently got her molars, but they're all poked through, and they were all through when she started her new sleep pattern. I try to take her out to run the halls of our condo building a few times a day to burn off energy, but it makes no difference to her sleep pattern if we don't go out at all. Today I'm going to try not putting her down for a nap, since she doesn't really nap anymore anyway, but that's really the only thing I can think of that MIGHT help.