I too get this problem with my flash drive & my macbook pro. What I found to work is, plug the flash drive into computer. Wait a bit. (I’m not on my mac right now, so I am recalling this from memory, bare with me) At the top of your screen on the desktop, click on ‘Go’ & then scroll down to ‘utilities’. It will open a folder filled with programs. Find the one called ‘disk utility’, click it. It should open up a window that shows you every storage device plugged into the computer, hard drives, flash drives, disc drives. Hopefully, you will see your flash drive in that list. If so, click on it & see if it says anything about, not functioning properly. Now, your best bet on retrieving the information will be a risky one. To get the drive functioning properly, while in the disk utility, click on the flash drive & choose, ‘format’. This will reformat the drive & will hopefully get it working. A good format is FAT32. Now onto saving the information. There are tons of programs out there, more for PC than Mac that will find information that has been deleted. I once did this for a flash card that was used in a camera. It not only found the photos I deleted just a minute ago, but it recovered over 5GB of photos…. the card was only 2GB. So those programs are very effective.
A small issue, when buying flash drives, I recommend only two brands. A-DATA, & Kingston. They have the best warranties, lifetime. I own about 8 flash cards, all from A-DATA & they are excellent.