What happens if you put the SD back into the adapter? will it show up on your computer again? I'm thinking that a couple things could have happened: one is the flash memory was corrupted when being removed from the computer. Kind of like when you yank a USB drive out without "ejecting" and you run the risk of damaged data. The second, probably less likely, is that an actual physical part of the card was damaged (perhaps one of the metal connectors on it or a miniscule amount of damage to the structure of the card- they're not the sturdiest things). I'm not sure how your adapter thing works- do you have to eject the adapter?
If the computer can read data on the card at this point, it's probably something between card and phone. if the card appears empty, see if you can add data to it via your computer, remove it properly from the computer, re-insert it and see if that data appears/can be read properly. If it can, then, again, perhaps the phone is to blame (or the little port where the card goes into the phone). If the card shows no data or the data can't be read then the card is probably damaged/corrupted.