It is extremely important not to lose your recovery sheet: you must keep it in a very safe place. If anyone gets access to it (and understands what it is), they can steal all your bitcoins (no need for PIN or security card).
However, if you were to lose your recovery sheet, immediately transfer all your balance to a temporary wallet. Then reset the Nano and create a new seed. Once done, transfer back your balance to your Ledger Wallet.
You can find the Recovery Sheet to download on this page.