You need the wallet.dat file. You could have either saved it from under %appdata% or in the wallet hit the backup button.

If you did not do this and have lost your wallet, then I believe there is no way to recover your coins. It is a harsh lesson, but you should always encrypt and back up your wallet to multiple locations. I save mine on 2 pcs and 2 USBs and they are in different locations also, in case something happens to my house.