If a wallet file is truly lost and there is no ability to recover it, the coins in it are lost forever. The Bitcoins do not disappear, but if a wallet is lost, there is no way to actually use those Bitcoins. So the coins are in effect removed from the economy since they cannot be spent. It is important to periodically backup a wallet.dat file. It is best to also encrypt those backups lest they be compromised giving someone else access to them.