It is hard to tell from here what is wrong. Did it ever start ok? Or was it bad from the start?
I have a Sandisk 4GB card, and it worked fine for a while. Then one day I was testing a forum user's code (it didn't disable the SD SPI) and I forgot to remove the card from the slot. The w5100 SPI communication destroyed the format on the SD card. I had to reformat it before the SD library would recognize it again.
I know fat16lib will disagree, but I used a Windows format utility to reformat my card and it did fine. However, fat16lib recommends a specific formatting utility. You may want to search the forum for that utility or maybe he will post here with that info.