I'm having some problems with me camera?

I have a 2 year old (almost 3 year old) Nikon D3100.It was working fine until recently. When I take it outside during the day it will say the subject it too dark to take a picture of but when the picture is taken its fine, it will even show up on the screen and through the view finder that it is showing up dark when its as in the middle of the day. I tried fixing the aperture, shutter and iso but nothing, I don't know what is wrong I don't think its the manual but I haven't looked. So please help me!

The only suggestion I can make is to get in touch with Nikon. If it can be fixed they'll do it but it will cost you.

If the view is dark through the viewfinder but the picture is OK then is sounds like the lens aperture blades have stuck. Look into the lens while it is mounted on the camera. Can you see a restriction in the lens with a small hole in it? If so then the lens may be faulty or it could just be a dirty contact.
Take the lens on and off a few times and clean the contacts with a microfibre cloth. If that doesn't work then a trip to the repair shop is needed.