>Finally, I was able to remove it in one spot but then I looked >in the viewfinder it was a lot of dirt on the screen and >viewfinder was dark. >The reason it was dark is because there is a lever that got >stuck just below the mount. I touched this lever and it sprung >back and viewfinder became bright again and I was able to take >few hundred pictures. > >Now, I need to take the camera to Nikon for a check up but I >can't reproduce most problems I have.
The lever is the aperture control for the lens. When a lens is being mounted, if the white alignment dots aren't fully lined up before the lens is pressed against the mount surface and twisted to lock it in place, it's possible for the aperture actuator lever to be dragged and jammed. If the lever is jammed in the stopped down position, very little light will come through the lens and then through the pentaprism and into the viewfinder. It will be dark.
The dirt you're seeing when looking through the viewfinder has accumulated on the focusing screen. It can usually be easily cleared using a rubber bulb blower such as a Rocket Blower. Don't use compressed air or a swab of any kind because it's too easy to mar the exposed surface of the focusing screen. It's also possible that some dirt has accumulated on the mirror, and it can be blown clean with a Rocket Blower as well.
Generally, the exposed surface of the focusing screen doesn't attract dust and detritus, but if a DSLR body is regularly allowed to sit for too long (e.g., more than half a minute or so) each time a lens change is done, dust and other bits can build up on focusing screen, the mirror, and the sensor filter.
If you've been able to take a few hundred photos since releasing the aperture actuator lever, and if the photos are turning out as expected, there probably isn't any need to send the lens away for service. Also, a jammed actuator lever on the lens doesn't mean the camera body needs service.