As a quick fix, you can use a rubber band to keep the viewfinder in. This covers slightly the back screen, but you should be able to still see enough. I found a small independent repair shop. They opened the camera, and replaced the spring in the viewfinder. They told me it was rusty, due to excessive use in wet/damp conditions (could be well true, as I used the camera also during all sorts of hiking trips). It costs me only 60$, much less than the original Sony repair shop.