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I would advise you to do a Reset All, or Factory Settings Reset. Some of your own custom settings will be lost, but the camera will be returned to its factory-released condition as far as settings are concerned.
If the Viewfinder is still blurred at that point (and the pictures taken too, presumably), then you have a main optics problem in all likelihood. The next question would be: But is the camera still covered by Nikon's warranty?
The answers to these questions would be the means of assessing whether the warranty has been voided or not:
How old is the camera? in months or years since you bought it.
Has the camera been dropped or otherwise subjected to mechanical shock(s) in its lifetime?
Has the camera been operated in or subjected to, temperatures below 32 degrees F (freezing point) or above about 100 degrees F ?
Has it ever been subjected to a humidity higher than 85 % or gotten wet ?
Has the camera been operated in a gritty or sandy environment, such as a beach, a seaside, a construction site, or the like?