Viewfinder is blurry with Nikon D3000?

I just got back from a trip abroad where I had been using my camera and it was functioning perfectly. I stored it safely, but when I returned the viewfinder was a little messed up. Whenever I look through it, everything is blurry, but photos still come out fine. I can still refocus both manually and automatically and zoom and everything, but the optical viewfinder is just very blurry. Also, when I focus, I can notice the image in the viewfinder similarly getting more or less blurry, but its normal "focused" stage is still very blurry. I've tried switching lenses which doesn't seem to work. As I said, photos come out perfectly fine. What could the problem be and how could I fix it?

Had a similar problem with a slr camera a few years ago, happened after a stopover in Malaysia… It was a type of mold on the inside of the glass

There's a diopter adjustment around the edge of the viewfinder. Point to a bright car outdoors during daytime. Autofocus on it. Look through the viewfinder then slide the diopter one way or the other to match your vision.