Ah, the lovely toddler sleep regressions. All three of mine have gone through this to one degree or another. I currently have the middle one (4) in her bed next to my bed whilst she's going through night wakings/shoutings. My oldest was like it until he was about 3.5 - still too young to rationalise anything about bedtime is bedtime etc, not time to be up screaming. He's 5.5 and now finally sleeps through the night.
What helped with him, and all children are different though of course, was a Gro clock - but only when he was really old enough to "get" the concept of star equals nighttime, sun equals wakeup time. It took a few weeks/months of constantly "it's not wakeup time, sun isn't up" for it to start to sink in.
Otherwise, for all of them, it was a case of go in, reassure them, come out, leave them for a bit, go back in, "bed time", come back out, leave it for a bit longer, and so on. Then the next few nights, leave it for longer and longer. When the youngest now wakes and shouts for me in the night (and just a normal shout, not bad dream shouting) I listen for a few minutes but she goes back to sleep because she's tired enough to do that. My oldest was different in that you just couldn't go in to him - once you did, it would make it horrifically worse if you left so he would get a quick kiss, "sun's not up yet", and leave and that would be it.