Two year molars was my first guess?

Night terrors is my second? DS1 had night terrors and would cry through the night for about six months after his second birthday. It took a few months for us to realise that going into him was actually waking him up and while the noises he was making sounded pretty awful, he was asleep.

Possibly sick?

It sucks when they don't settle and keep waking all night long. I hope it all sorts itself out quickly for you.