Why my image is blurred?

The image suddenly blurred out, or shall I say not as sharp as before. Some are over-flashed, some are too dark, some are just plain white. I just can't figure it out why as I've been using this camera for quite some time already and never encountered this kind of problem before (this is my first time facing this problem). Recently, the image i see through the viewfinder is very blurry - blurred words, blurred people's face, etc. It's just not as sharp as before and I noticed a black dot on the viewfinder. No matter how hard I've tried to wipe it off, it is still there. I thought it's because the film already expired, but it still happen after I changed the film. Please help me out as I don't know what to do and I really love the camera. Please help me.