Losing save data when moving to a new console is one of the biggest problems for users.
Possible reasons:
Saves were stored only locally and were not transferred
Nintendo Switch Online cloud backup was not enabled
Data was not migrated using the transfer function
Ways to restore:
If you have a Nintendo Switch Online subscription — saves are automatically synced with the cloud. Simply log into your account on Switch 2 and download them.
If you didn’t have a subscription — try local data transfer: on the old console, enable Data Transfer (in the System menu), then perform the receive function on Switch 2.
If the saves existed only on the old console and it is lost/broken — restoring the data is almost impossible.
Some games (for example, Pokémon) use their own servers to store progress — check your game account.