Check if your 3DS SD card happens to be corrupted as this will cause the 3DS not to be able to read the files off it. Take the SD card out of the 3DS and insert into your computer. For Windows desktop/laptop, see if Windows Explorer can read and access the SD card.
If you are able to go into the SD card, copy everything off the card and onto your computer, set aside. Quick reformat the SD card by right clicking the SD's drive icon / name >
format... , File system = FAT32, Allocation unit size = 32 kilobytes, and
Quick Format checked off. After reformatting, copy everything back onto the SD card.
*PLEASE NOTE: If your SD card has a memory capacity larger than 32 GB (gigabytes), you cannot use Window's File Explorer to do a FAT32 reformat as it will not create a complete partition of the whole card. Window's File Explorer will only format up to 32 GB. You will have to do a FAT32 formatting using different program, such as the free to use
SD Memory Card Formatter 5.0 by the SD Association @ sdcard.org [To Mods, please redact this info if this goes against Rules/ToS].
If the SD card shows up corrupt on Windows, you're tough out of luck and gonna have to rebuild everything from scratch. Sorry bud.
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Edit : Another problem that is rare but does happen is that your SD card has physically died due to read-and-write wear-tear. A semi-failure mode is that the SD card will enter into Read Only Mode and will not allow anymore Write Mode. To check if your card has possibly failed or started to die after you have copied everything over to your computer, right click on the SD icon / drive name. Go to
Properties >
Tools >
Error checking > Check.
Edit #2 : I can only speculate that if you are using Luma3DS, boot9strap, ntrboot, AND that your SD card is fine with intact files, check the Luma Chainloader by pressing and holding the select button at 3DS power up. See if
Luma3DS v9.0 configuration has the
(X) Autoboot EmuNAND selected. This is in case you have all homebrew stuff show up on EmuNAND and not sysNAND.
Another problem that may happen is if you used a secondary SD card and your 3DS sysNAND ended up reseting all the installed title tickets of your games on Home Menu. Check
System Settings >
Data Management > Nintendo3DS > Software. If any of your games appear with crossed out x's, your title tickets are missing. Some of these can be restored with the
TIKDevil app.
Edit #3 : If all else fails, as cliche as this sounds, resort to the 3ds.guide to redo everything. Yiks.