First, you know what system firmware version you're on?
I hope your sd card isn't completely dead otherwise your data'll completely gone ( especially if you don't have any backups)..
Any you'd want check this new sd card with
h2testw to make sure it's genuine, format as Fat32 32kb cluster size before and after the test. If it passes the test, follow
this to copy the latest version of
Luma3DS to the root of the sdcard and with that you can access the home menu (note that you won't see any of your installed games/hb etc, that data is on your old sd card). If it doesn't pass the test, then it's fake, and you should get another replacement that's genuine.
As for the old sd card, insert it into your pc and see if you can salvage any data (if you don't know what to look for, just post a pic of the files/ folders that's on the root of the sdcard). And do not format it if the pc doesn't somehow recognize the sdcard, as doing so will wipe the sd clean.
