What does it mean when I turn my camera on and it says card cannot be accessed. Reinsert/change or format card with camera? I was taking pictures turned it off and when I turned it back on to take more this came on the screen.. When I got home I tried to downloand the pictures and say card is empty. Could the card of gone bad ? I need help. Thanks
Don't use the card (if the photos are important). Use a SD card recovery tool:
Are you sure you took the photos in the first place (did you preview any of them)? If so, I'm not sure what happened, you didn't touch the card at all? I've never had this problem *knock on wood*, but have heard of a lot of success stories using recovery software.
Flash / non-volitile memory cards actually "wear out" and are not intended to last forever. The process of "reading" from a card does absolutely no damage to the card (you can read a card an infinite number of times), but the process of "writing" to a card is technically a distructive process. Each time you "write" data, it very slightly damages the card. This has to do with how non-volitle flash memory works.
Better cards will handle thousands of writes (often hundreds of thousands) before it's a problem, but all cards WILL eventaully wear out if used enough (that's a certainty based on how the technology works). You should think of these things like the tires on your car. After each drive you wouldn't necessarily notice that the tires are wearing out, but you know that if you drive the car enough, you'll eventually need new tires -- and it's not a defect of either the car or the tires, it's just the way things work.
I tend to stick with the better card technologies... since a bad card can ruin your whole shoot, it's not an area where I'm willing to save $5 by buying the cheaper cards.
Lexar and SanDisk brands seem to have a pretty solid reputation.
Keep a spare card with you so that, should you get this error, you can immediately pull the card out (the ability to recover the card depends on your NOT reformatting or over-writing the card with new data) and slip in the spare so you can keep shooting. I always have a few spares...
I see guys use these huge cards, 32GB and more. I use smaller cards and change them as needed or if needed. This limits the amount of loss that you are exposing yourself to. It just takes one time for you to become a believer. Just like backing up your computer. After a loss you get pretty religious about it! Key word "after".
Use smaller, high quality cards and format each time before an important shoot. Have spares.