How to take pictures with my camera through my telescope?

Okay, so I have a nikon d3100 camera, and I have all of the components of attaching it to my telescope. But, the strange thing is is that I can take videos, but I can't take pictures. I have the right setting on my camera (M setting on the turning wheel on the camera) to use it, but for some strange reason I can't take photos that turn completely black. When I take the photo, the picture is there, but afterwards when I look at the pictures in the 'gallery' on the camera They are all black. There's no picture showing up. But, when I take videos it works really well. Anybody have any solutions? This will really help me as there's transition of venus next tuesday sunset and I want to take pictures for that.

It seems like a focus and/or exposure time problem.

You say videos are ok.
I assume they are videos shot in daylight.

And in any case always rehearse in daylight. Things are so much easier then. So focus on the most distant object in your field of view and take a photo.

And I presume "M" = manual.
If so, you need to either set an exposure time, or use a shutter release cable and operate the shutter yourself.

And at night, if the shutter clicks shut after only 1/500th of a second, you will just get a black photo. You need the shutter open for much longer to see any stars.

Daylight photography of the sun (with the black disc of Venus) probably needs special filters.

Time to head to your camera shop / astronomy club / photo club.

Many new cameras don't have a connection for a shutter release.

The M setting is manual mode. It allows you to set the ISO (sensitivity), f-stop (aperture), and shutter speed manually. Just setting the dial to M and not adjusting all the settings M mode allows is useless. You need to think like a professional photographer. I'm at a loss to understand how videos come out good in automatic mode, but maybe it kind of "learns" about the light conditions from the first few frames and adjusts itself as it goes. Are the first few frames of the video poorly exposed?