We went through something very similar about 2 weeks ago with my 22 month old. He even stopped wanting to go to bed. In desperation, I tried putting a baby-gate at his door instead of shutting it (which we always had) and leaving the hallway light on. That solved the problem. The first night, I sat outside his door until he went to sleep and when he woke-up I told him that I was right there. Now I just tell him that we'll be right outside and he's back to sleeping through the night.
Hope this helps.