Sounds like you might have accidentally underexposed your images somehow.
Are you shooting automatically or manually, as in are you controlling exposure by setting the ISO & shutter speed, or are you capturing automatically exposed images? If you are in Pro mode, there is exposure compensation that can be adjusted, you may have accidentally changed it to be under-exposed somehow. I don't know if you can under-expose in the automated image capture mode as I haven't used it for photographs.
