Just wanted to say that I really feel for you - my (19m) DD has been the same for the past three weeks.
Naps went out of the window and she would take ages to get to sleep then wake every night at midnightish and take up to 3hrs to settle. Before that she was great, 2 naps a day and self-settling at night and after waking.
Anyway - just wanted to say that for the past 5 days naps have been OK (after a little shouting it settles down) and she has slept 7 to 7 for the past three nights. (Hmm...hope I haven't jinxed it now and she is up all night!)
I know it might go wrong again but feel like I can handle that better in the future knowing that it won't last forever, even though it felt like it at the time.
I tried a combination of things depending on the night in question - sometimes I will let her cry a bit but will keep going back in. She stands in her cot and screams when I go out but I keep going back in and telling her to lie down; sometimes I did this soooo many times and it just didn't work but she seems to have got the hang of that for naps and I'd say I maybe go out and straight back in again 4 or 5 times by which time she is lying herself back down, still crying a bit but feebly IYKWIM then I can shut the door gradually and tiptoe out.
Other thing was to cuddle her until almost asleep then put her in her cot; she would cry, I would ssshhh and just gradually withdraw from the room - helped if it was pitch black so she couldn't see exactly where I was and she would gradually get more sleepy.
I really worried that I would be 'breaking' her good sleep habits and causing problems but so far she seems to be OK although not exactly as good as she was before. I think it was a combo of sleep regression and full-on teething (lots of molars coming through at the same time) and really felt different from any disturbed sleep in the past.
Had some good advice from someone else on MN to have 'quiet time' even if she won't nap - this does seem to have helped. Although she's moved from 2 naps to 1, we now sit down quietly around nap time each morning and have a quiet story and a drink. This seems to give her time to recharge a bit and makes the morning easier. I had been doing the opposite and having her running around, doing loads of stuff in an attempt to tire her out but I think it just exacerbated the problem.
I was sooo tired though and everything seems loads more difficult to cope with when you're exhausted. Any chance anyone would look after her for an hour or so and you can rest? (I didn't have anyone but wish I had). Otherwise while I used to lie on the sofa in DDs room cuddling her to try to get her to sleep, at least it was 15 mins of peace for me too.
Really hope it passes soon and you get some well-earned rest. Good luck!