cart or digital download? if its the latter, your sd card is probably corrupted. Dont think you can get it back, sorry :( If its digital and your sd card isnt a known quality brand, theres your problem :/

do you have the app called Checkpoint? if u do u can restore your backup, if you dont have it u should def install it now :( that way you can make backups of your games in the future

thanks for helping (i have checkpoint and a quality sd card) (its still lost though)