If it's recognised nowhere (not in a camera and not in a few PC's) it's gone bad. See if Windows disc management still sees it. If it does, try repartioning it. If it's OK in the PC, try to format it in the camera.
If no luck, try another card in the camera.
I recently tried to record something using my SanDisk Ultra 64GB Micro SD XC1 memory card I have used in the past and the camera itself (Sony Handycam HDR-CX240) kept indicating that there wasn't a card inserted. I reinserted the card several times making sure it was aligned correctly and pushed in all the way and nothing. I didn't have any data in that card as I had dumped it in my laptop to make more room for new videos and now it just won't even acknowledge there is a card in the camera. This is not a new card, I had used it previously several times without issues in this same camera. What can be the problem?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic