My son who had been sleeping very well since a baby, started sleeping horridly at about 27 months as well! It started out he was awful to get to bed and then was also waking up an hour or 2 before normal. It progressd to him waking up at night and not being able to get back to sleep without someone sleeping in the room with him. We tried everything, and although I do think a few things helped, it mainly just took time. I think the worst was over in about 4 months. What ended up happening was him not needing as much sleep, even though in the beginning it was rough. It was similar to when he dropped his second nap and was pretty crabby at first.
We did figure out he was pretty scared when he was having a hard time going to sleep and also waking up. We ended up having to leave his light on low and his room door open with the hall light on all night. We also had to reassure him that we would be back to get him up. Luckily, he is STILL in his crib at almost 3 years. For whatever reason he had lost his ability to sooth himself to sleep when he woke. For all the kids between 2-3 that develop awful sleep, I think there must be something developmental causing it all.
Ultimately what has happened is that we moved his bedtime back to 1-1.5 hours later than it was. And also not letting him sleep more than 1.5-2 hours for nap, starting no later than 1:30. We did give him the comfort at night he seemed to need when he was at his worst, but when he started sleeping better again we weaned him off it. I wish you the best of luck that this stage will quickly pass. It does get better!