If you are talking about SD Card in lgl34c mobile model then try below given troubleshooting methods:
Solution 1: Try rebooting the phone- Turn off the phone. Reboot the phone to see if SD card starts working again.
Solution 2: Repeatedly insert and pull out the SD card- Take your chance to try inserting and taking out the blank SD card for many times, by these attempts your phone may finally read the SD card.
Solution 3: Run Chkdsk to fix errors- Connect blank SD card with PC via card reader and make sure it is detected by the machine.Run CMD as administrator, and input chkdsk n: /f(n is the drive letter of the blank SD card) to fix errors.Put the SD card back to phone and check whether the card works fine after that.
Solution 4: Check hidden files on SD card- Connect blank SD card with PC via card reader and make sure it is detected by the machine. Run control folders to bring out the File Explorer Options.Turn to the View tab.Check Show hidden files, folders and drives and save the changes. Delete the "specialty files" in the blank SD card if there is any.
The above four solutions are absolutely perfect to fix your issue. But in case if none of above works then check Lg L34C Manual and exercise all steps given in SD card/ Storage Troubleshooting section.