#1. "RE: D3100 "This Memory Card Cannot Be Used"" In response to In response to 0
I use SanDisk Image Rescue, not sure what you've tried, but it's got a pretty good recovery rate for me - high 80s percentile.
As far as the card is concerned, it's time to take it out of circulation. You can try to reformat it in the camera, but once it's failed, you don't want to take a chance on it, especially with images that are irreplaceable in any sense. Given the relative low cost of SDs on the market, just replace it at this point and chalk it up to operating costs.