First, the "r06" message is NOT an error code. It indicates the number of shots remaining in the internal memory buffer, before the camera must prevent additional shutter actuations so that it may transfer those images already in the buffer to the memory card. This "r" number will decrease with each shot taken in rapid succession until it reaches 0. The shutter release will no longer work until the buffer is transferred to the memory card, then shooting may continue again.
As far as not being able to even see through the viewfinder, it sounds like something may be obstructing the light from reaching the meter and viewfinder. The problem is most likely the position of mirror inside the camera body.
With the lens off the body, the mirror should be plainly visible at
about a 45 degree angle to the opening. A side view drawing of this is
below. The solid red line is the mirror in the normal position. The
red dashed line is the up position of the mirror when the shutter is
When the mirror is in the "normal" position, the light from the lens is projected on a
screen so that the image is visible in the viewfinder
for composing and can be metered. When in the "up" position (when the shutter release is
fully depressed), the light from the lens is projected on the camera's
sensor for as long as set by the manual settings or program; based on ISO,
aperture, etc. At the end of this time, the mirror
returns to the "normal" position.
If your D100's mirror is not in the lowered 45 degree angle position,
the image seen in the viewfinder is inside the camera - not that which
the lens would project. Hence, the dark viewfinder, long exposure times
and - I'm guessing - severely overexposed pictures because way too much
light is striking the sensor because the meter is only seeing darkness.
If the mirror is ok, with the lens removed from the body, look for the aperture lever as shown in the yellow circle in the picture below:
By default, the aperture is at minimum. The camera moves the lever from this position to full open (and anywhere between) as needed. You should gently move the lever to the other end of its travel to open the aperture to maximum. Point the lens away from a light source but preferably at a light colored background. Look through the lens. There should be no obstructions and be clear. Next, look at a dark colored background to find the same results. If you want, you can even allow sunlight to shine through the lens onto a sheet of paper (like a magnifying glass). The result should be a bright circle with no obstructions. Obstructions in the lens will prevent the meter in the camera from getting accurate information about the scene and if significant enough, prevent viewing through the viewfinder.
You may wish to have the camera & lens professionally cleaned and serviced to repair a mirror or lens issue.
If this was helpful, please rate it as such. Good luck!