Pictures too dark. Shutter operating too slowly.

Did you switch to Tv (shutter mode) and took pictures for several seconds? like in the dark? Because saving the picture can take as long as the shutter time was.
Before panic. (Actually you never should panic) check your camera in automatic mode. And make sure you did not select any power save action, that puts your display on dark while shooting.
But always be sure your battery is charged and check its capacity. In the menu yo can check under the wrench symbol, when you select Battery info. Page 268 of the manual.
Working with a bad battery can damage the camera in the end.

What mode are you shooting in? It seems you may either have your shutter speed set manually to a slow speed or, alternatively you have set your apperture set to a high number e.g f/20 or there abouts. Because this is a very narrow apperture the camera will compensate by using a slow shutter speed.
Try some shots in fully automatic mode and see what happens. With a slow shutter speed you can expect any movement to produce blurred images.

a battery will not hold full charge when its not being used it will slowly lose charge over time keep your battery in a dark cool place if it is connected to your thinkpad it will gradually lose charge
hope this helps

If you have the camera on continuous then it may be that the charging is not keeping up with the shutter. Check the recycle time on the flash and compare with the burst speed on the camera. The burst speed on the 5D is fairly rapid.

Sounds like a loose screw inside shorting out one of the curciut boards! tccamerarepair.com

Make sure your battery is fully charged. If the battery is not fully charged your flash may not fire at the brightness you need to take a well exposed photo. Also, check your ISO settings. Try taking photos at a higher ISO when the subject is in a dark area. Experiment with the ISO. Good Luck!

LENS SHUTTER OR F PCB OR MIGHT CCD PROBLEM

Both of these problems are caused by underexposure. There are several causes for this. If you are set in AUTO MODE check the shutter speed and lens opening in the view finder. If the speed is less than 100 and the shutter is less than 5 and you are not using flash. You will probably get blured and dark pictures.  Practice shooting with your settings outside in sunlight , inside with lamplight, inside with flash and all of the above with motion. Check the exposure after each shot and make adjustments. In a short time you will get very good at setting your camera.

First u have to reset the camera,go through the menu and select reset and yes. the camera will take default settings.Try now.

Reset the camera opening the connector cover.

You try with a different compatible lens .If works we have to change the lens.

Remove the battery and get fully charged .Operate all switches for some clicks. then put the battery and try.

Then u have to consult a mechanic.

I have a sony cybershot DSC-S650, the LCD was very dark and could not see anything except if I pointed it at a bright object, everything else worked ok. I could take pictures but could barely see them in playback as well.

I took it to Sony authorised repair outlet and was told the CCD was damaged, they charged me USD $55 to repair it (in Quito, Ecuador)