Lenses not working? usually too dark sometimes too light

When looking at the screen to take a picture the picture is dark. As I take pictures One picture the lighting will be fine and the next will be black and the next can show up as really bright without me changing lighting or where I'm standing. It's getting worse it is pretty much always dark now. Doesn't not matter what setting the camera is on. I hope that makes sense, kind of hard to describe. Also at times when I hold the camera to take a portrait shot the screen only shows black but when I turn the camera to landscape it is fine (although it doesn't do this anymore because it is always showing black). I'm wondering what the problem is? Is it fixable and how much? The camera is 4 years old.