Well, first I would have to ask, why did you take the memory card out of the phone? In most cases, unless replacing the card or resetting the phone, or the phone got wet, there really isn't a need to remove the card.
If you are removing it to read the data from a different device, like say a computer, then you would do better to leave it in the phone, and connect the phone via USB. If, however, you are removing the SD card to use it in a different device, camera, computer, etc., then it is completely possible that it was reformatted by that device and will not work in your phone due to that change.
Or, your card could be just unusable at this point. That can happen if it is taken in and out of devices over and over. The card slowly...