I CAN't GET MY PHOTOS OFF THIS SD CARD?

I had a memory card in my nikon d3000 with a few photos on it. Today i took that same memory card and did an entire shoot on it with my nikon d90 and now i can't get the photos off the card. Lightroom detects the files but can't preview them or export them. I thought nothing of it while i was shooting because the photos were still writing to the card. Any suggestion would help

Use your computer skills to copy all the image files from your memory card to your computer to a new file folder named for the subject/location and date.

Then import them into a new Catalog with the same name.

It does NOT matter which camera you use, but you do need to get used to using different memory cards for each camera since your camera will create a new file folder name on the memory card based upon the camera you are using.

ALWAYS start your shooting sessions with a freshly formatted memory card, formatted on the camera you are using.

Use a USB card reader rather than a camera cord (costs peanuts and your pc may already have one anyway.)

If it still can't get at the file structure then use free file recovery software.
http://en.softonic.com/s/full-version-sd-card-data-recovery-software

Hereafter do as fhotace suggests.

The cameras should have created and written the files to different folders.
Are you using MyComputer a.k.a. Windows Explorer to retrieve your files? That theoretically should work okay. [But maybe not.]
If you are trying to import them into a program like Lightroom that might be more likely to get confused.

Time to learn a new trick. Each time before you do a shoot, format the card. Care for another trick? At the first sign of trouble, throw away the memory card.

Now for your problem. Use Recuva. After getting what you can (no guarantees you'll get all), throw away the card.