If you are trying to access the SD card using a USB card reader that connects to a USB port on the computer, try the suggestions below. You can also try using an external USB memory card reader instead. You would need to have an authorized service center look at and repair or replace the memory card reader in your computer. If the other memory card is also not readable by the built-in memory card reader, it is a string possibility the memory card reader is not working. If it can be read, your memory card is bad and likely needs to be replaced. If you have access to another memory card, insert it into the computer reader to see if it can be read. The memory card reader may not be working properly and causing it to not read the memory card.
Some laptop computers and desktop computers have a built-in memory card reader. If you're experiencing this problem, visit your phone manufacturer's website for support. If your smartphone cannot read an SD card, the card may have been formatted by a device that's incompatible with your phone's operating system.