If your wallet is not hierarchical determinstic (HD), then there's a pool of 100 keys that are pre-generated and stored in the backup, so the next 100 addresses you generate after making your backup will protected as well. If it is an HD wallet, then every address (not just 100) is protected in this way. Try generating an address in your wallet and it should match the one you generated last time.
Thanks for the idea. I generated a couple of new addresses, but all of them are completely different..
I initially created wallet.dat many years ago with an old bitcoin core version.
Unfortunately the address has not been properly backed up then. It was vital that non-HD wallets were backed up regularly, and the documentation didn't stress this clearly enough in my opinion. As jackg mentioned, there might be ways to perform data recovery; you should
immediately stop using this computer if there is a chance that the file was deleted, but pieces of it still remain on the hard disk.