I was taking some inside pictures with my Nikon D3100 and 18/55 lens when, all of a sudden, the lens started to work funny -- the more I zoomed in on the subject, the fuzzier the picture looked in the viewfinder. I shut the camera down. The next day I tried it again and saw the same effect. Then I got the idea of seeing how a picture would actually look on the LD screen, and just as suddenly, what I saw through the viewfinder was normal. Things were OK again. What happened?
I bet you accidentally adjusted the viewfinder diopter, which lets you compensate for minor or moderate vision problems without glasses or contacts.
On the D3100, it's a little dial above and to the right of the finder window.
Try this to get it set right: How do I calibrate the diopter correction on my DSLR's viewfinder?