Q:

Nikon d5100 producing extremely underexposed images

I have a most vexing problem. 
I have a Nikon d5100 (roughly a year old) which up until two days ago handled exposure and metering just fine. However, at night, in a well lit room (florescent bulbs) in which I NEVER had problems capturing images at reasonable shutter speeds, apertures and ISOs (< 400), here's what's happening now:

In Manual, even at low shutter speeds (1/90 and lower) and relatively open apertures (3.5) - ISO 640 and less - my pictures are almost completely black. Pointing the camera at a bright lightbulb just barely captures the light with heavy shadows all around it.
Exposure compensation no longer has ANY effect on the exposure
Shooting in Aperture Priority mode at f/3.5 at anything lower than ISO 1000 reduces my shutter speeds to 1/2 or lower to compensate.

Yes, I know. ISO. Keep in mind, the room is quite brightly lit and I never had this issue prior to last night. That being said, the only way I can capture a reasonably exposed picture here is at ISOs 6400 and above. But even a picture at 1/125, f/5.6, ISO 6400 (manual) is still ridiculously dark.
I have taken the following step to troubleshoot, to no avail:

Turned exposure compensation up to the max +5.0 - no effect
Turned shutter speeds down to 1/40 and below - extremely limited effect
Turned ISO up to 6400 and beyond - limited effect
Update the camera's firmware (1.01 > 1.02) - no effect
Reset the camera using the buttons with the two green dots - no effect, multiple tries
Messed with the metering modes - no effect
Turned it off and on again - no effect
Stared pointedly at the camera - camera didn't even flinch

I am at an absolute loss here and I really, truly, need help.
Images:
ISO 100, f/3.8, 1/3

ISO 100, f/3.8, 1/90 

ISO 1000, f/3.8, 1/90 

A:

it is with shame in my eyes and regret in my heart that I answer my own question: 
All my problems stemmed from a single setting change. I looked back at the metadata of pictures I had taken in the same location/circumstances and was quite surprised to see most of them in the region ISO 6400. Now, I rarely set my ISO that high so imagine my confusion. Upon the advice of a friend, I dug through the menus for a bit and found a setting called "ISO sensitivity settings" and there I discovered that "Auto ISO Sensitivity Control" by default had been on. In summary, for the year that I've been shooting with this camera, auto ISO has been enabled, so even if I had set ISO 100 from the settings screen, this buried menu option would override it - hence the large number of low light pics in my library with very high ISO.
Despite my own inexperience being at fault, I truly appreciate all the excellent advice you fine people have given me. I will leave "auto ISO" to rot in the dark all by its lonesome for now, and learn to use my camera properly

