4 gb sdhd memory card

Hello,
i have a samsung gravity 3. i bought a sdhd memory card for my phone. first few days it was working fine. then all of a sudden it stopped working for me. my phone won't recognize the memory card and the computer. everytime i tried plugging it the file is not to be found. my sister has the same exact phone and memory card. its working fine for her. i tried put other memory cards and it will show up on my phone. i triedd all of these suggestions but still my computer nor my phone will recognize it. ): help please