Can't transfer pictures from my nikon d3100?

I took some pictures at a wedding last month and put them on my computer and all but I couldn't upload them but there still on my cameras SD card when I put it in the camera but when I go to print them using the SD card it says there not there help

It seems that your workflow is flawed.

After each shooting assignment, you need to insert the memory card into a card reader and then using the computers file manager, copy all the images to a new file folder named for the subject/location and date.

Once they are all safely on the hard drive, the memory card needs to be formatted using the format feature on the camera.

Since this is an actual assignment (wedding) the next thing you need to do is make a full back up DVD of all the images so that if something happens to your hard drive, you still have copies of the original image files.

It seems that you are trying to use the picbridge feature on the camera.

This is not the optimum way to make prints from any event let alone a wedding where you heed to make sure that the white balance is near perfect before having prints made.

Using your copy of Photoshop CC or Lightroom 5, do any cropping you need to do to make the size prints that were ordered (you need to crop images so that the match the image aspect ratio of the paper you are using). When you do crop the images, ALWAYS save the cropped image as a new file name so that you do not overwrite your original images.

You should know all this before you start taking on ANY kind of photo assignment, but especially weddings.

Before you go any further, do this.

* Burn a backup copy of all the images of the wedding on your computer and put them someplace very safe.

* After you have done any processing of the images, use Photoshop to print out the images using your photo printer. If your computer monitor has recently been calibrated and you used the correct white balance settings during your shoot, the prints should be excellent

Remember to have Photoshop manage the colour when printing, NOT the printer.

I'm confused! You say you "put them on your computer" but you "couldn't upload them" Surely, if you put them on the computer, then you DID upload them?
Anyway, if you are connecting the camera to the computer, then stop. Instead, buy a Card Reader. You pop the SD card into one slot and the other end goes into your computer USB port. It works every time and is so easy to do.