In one bedroom, a couple of years ago, a small thin area of plaster came off the wall. The area is a above the window and felt damp. My DD used to sleep in bunk beds and she put her hand up and noticed that the area above the window was damp. The plaster just crumbled off.

I want to re-paint the room. How do I fix this problem? I don't just want to re-plaster it as that would just come off too, no doubt. What can I do to the small patch where the plaster came off, before re-plastering and re-painting?