If your photos are sharp but the optical viewfinder looks blurry, the most likely cause is the viewfinder diopter adjustment being moved. The diopter changes how the viewfinder appears to your eye; it does not affect the actual focus recorded by the sensor.

Check the eyepiece and then adjust the diopter wheel beside the viewfinder while looking at the viewfinder display information (focus points, exposure data, etc.). Turn it until those markings look sharp to your eye. That is the correct way to set it.

Also make sure the viewfinder eyepiece itself is clean.

Cleaning the image sensor or front lens element usually will not fix this kind of issue, because neither is what you are looking at through an optical DSLR viewfinder.

If the diopter and eyepiece are fine and the problem started suddenly, there could be physical damage or a misalignment in the viewfinder path, such as the mirror or focusing screen. In that case, have the camera checked by a service center or knowledgeable camera shop.