I turned my 3DS on and all of my extra data was missing. :/
Is it too late to save any of it? I lost Netflix, my Pokedex, all of my Ambassador games, my trailers. I know I can redownload them from the Eshop, but I don't feel like having to beat everything again.
Check the SD card, is anything missing? Try backing it up in a folder, if you can't back anything up you lose all of your save data. Buy a new SD card, put it in your 3DS so it can format, then you can move all of your old stuff over to the new card.
Remove and put your SD card back in, sometimes it get jiggled and pops out of place. Put your SD card in a reader and check if everything is there. If you still don't have your data you should buy a new SD card or this may happen again.
Check your system data page in the system settings. Sometimes the information stays even if the game is gone from the SD Card memory.
Try what Kaiaa said before spending money to replace the SD card.
I'd also check all your memory and try backing up the SD card memory into the internal memory.
