Did you close the software or turn off the console without saving? Or did the console run out of battery before you could save?
→If you close the software or the console loses power before you've saved, any unsaved progress will be lost. Please make sure to save before closing the software or turning off the power. Some software includes an auto-save function and will save automatically.
Are you using your game card in a different console from usual?
→Your save data is stored in the system memory of your Nintendo Switch console, not on the game card itself.