I looked for a previous post about this because it comes up from time to time - but apparently it's been long enough that nothing shows up anymore. I'm sure it's an unnerving experience to lose digital images but somehow I haven't managed to do it (yet). If you have any tips to share, post them here. I'll start with what I know (which is what I'd do in this situation):
First, don't do anything to the card just yet. Don't try to look for images with your PC, don't put it back in your camera, don't format it or (especially) shoot with it. Check your options, try data recovery software and then follow it's instructions.
I've heard good things about this software. There's even a free trial. If it doesn't work, you might try a different program - I've heard that if one program doesn't get it, another one might.
If that doesn't work and the shots are important enough that you want a professional to try to recover them, then you might want to talk to Tallyn's in Peoria IL. They offer this service - it's expensive but if the shots are important it's a better option than telling your client you lost the images!
So how do you prevent this from happening in the future? Well, the best thing I can recommend is to always format the card in the camera it will be used in every time before you use it (obviously downloading everything from the card first). Don't format it in the PC or in a different camera. Don't turn off the camera while it's still writing to the card. Don't take the card out of the camera or a card reader while it's still reading or writing to the card.
Any other tips out there - or horror stories and what worked (or didn't)?