SD card gone kaput? Help?

I've got a SanDisk 512 SD card, and I just got a Nikon D50 DSLR camera.
I took it out about 10 days ago and took 200 pics, some of them
actually GOOD. Took the SD card out, and with a
USB-based-SD-card-reader I look at 'em and showed 'em around and so on.

I left the SD card sitting in the USB reader sticking out of the USB
port on the outside of my PC (WinXP). This may have been my mistake.

Today I went to move the pictures onto the hard drive, edit 'em and
post 'em on flickr. I can't. The Removable Drive shows up in My
Computer, but I can't open. I can't right click on it. It just goes
into deep slowdown mode and often requires me to force-quit
Explorer.exe - or else times out.

I tried using another card (in fact, a memory stick) in the same USB
reader and that works fine. It's not the reader. I put the card back in
the camera. Oh-oh. "This card cannnot be used"

I tried this on another PC and it's the same thing.

Have I trashed the card? Was it from sitting out? If Windows won't
recognize it, I can't even reformat it, let alone get the pictures off
of it?

Any ideas or experiences here?