Lets here from other, experienced moms, here. Its really common around this age and older, for children to develop fears of the dark and of going to sleep alone, often expressing fears of monsters or bad people who might harm them. When you describe her as frightened that you open a particular cupboard door, and warns you, fits perfectly with this. Fortunately this is within the normal range of chioldren's behaviour, and there isn't the faintest chance that there are ghosts or spirits involved.
This is often a passing phase, and it can help to use loads of confident re-assurance, a night-light ( a dim light in her room that doesn't produce scary shadows and shows there's nothing nasty lurking there ; maybe in the daylight show that there's nothing in the cupboard, and discuss with her if she'd feel safer if it were left open so nothing could hide there, or if it was locked so nothing could get in or out.
A quick fix can be to allow her to sleep, if possible in her own bed, in her parents room for a few days - the problem then can become persuading her to return to her own room !