I would immediately save all data from the card to the computer. Then try another card (a known good one) on the phone to establish whether the problem follows the card or the phone.
If the problem follows the phone (the known good card does not work either), then I am sorry.
If the problem follows the card (i.e. the known good card works with the phone), then examine your card for any physical damage. If found, get a new card. Never use a damaged card. If no damage, try to re-insert the card a couple of times.