Plaster has come off the interior wall

[Totally_Confused]

We renovated the lower floor of our split-level house about 5 years ago. We have a hallway wall where the plasterboard (or whatever the material is) has come loose from the wall. Does anyone know how to fix this, is it difficult or can we do it ourselves?

 

[kniv]

It's plaster, not gypsum. Knock off everything loose, fill in, sand, paint the entire wall.

 

[Dede77]

Just as @kniv writes, it's plaster.

However, it might be a good idea to check why it's coming off.
It looks like an inner wall, but check to see if it's damp.
Are the walls painted with a paint that can allow moisture to pass through?

 

[MathiasS]

I would chip away what is loose and then mix a bucket of plaster and restore. It is quite easy to work with for an enthusiastic amateur.