Why aren't my pictures showing up on my camera?

I have two cameras: a Nikon D3000 and a Sony SteadyShot. The memory card reader on my laptop doesn't work anymore, so I used the USB cable to hook up my camera to my laptop to upload pictures. I took the memory card from the D3000 (a 4 GB SanDisk Ultra) and put it into my Sony camera to upload the pictures using the USB cable for that camera, but only two pictures showed up: the first picture taken on that camera and one from December. I put the memory card back into the Nikon and took it to my desktop computer and used the Nikon USB cable to plug it into that computer, but only the first picture showed up again. I'm getting really worried, because there were over 250 pictures on the Nikon, and they're all very important (priceless family pictures, photos from plays I've done, etc.) and a lot of them I haven't had the chance to develop yet. Could anybody tell me what went wrong? And how I can get my pictures back? :$

Added (1). Also, when I now turn on my Nikon with the memory card in it, none of the pictures turn up except the first one.

You're missing the obvious answer, one camera can't read photos taken by a different camera.

Get a usb card reader that should take care of the problem.It'll see all files taken by any camera.

Each camera has a unique space on the SD card. There's a folder called DCIM, and under that folder, there will be a folder for each camera. The Sony files will be in one folder and the Nikon files will be in the other.

The thing that has me concerned is that when you put the card back in the Nikon, those files should show up again. Did you look and see if the camera itself could see the files (not using the USB cable).

And if you did anything when the card was in the Sony - format for instance, you will wipe out the photos in the Nikon folder.

First, get a new card reader - they are not expensive. In fact they are cheap.under $10. Sometimes they are even free when you buy a SD card.

Then try to download all of the photos to your computer using Windows Explorer - not the software that comes with the camera.

Finally, if photos are missing - do not use the card until you download Recuva and run it against the card. Most of the time, Recuva will restore the files, as long as you have not used the card to take more photos.

What Jim said, plus you may have corrupted the card with your musical cameras strategy. You may need to download a photo recovery program and, using a USB card reader, go looking for your photos.