What happens to my bitcoins if lose my wallet file e.g. I suffer data loss and my computer is unrecoverable?
If a wallet file is truly lost with zero ability to recover it, then the coins it contained are lost forever.  Think of it as cash left in a burning house.  

Be sure to periodically backup your wallet.dat file.  It is best to also encrypt those backups lest they be compromised giving someone else access to your wallet.