In multibit, under the help menu option you can open the help contents. Under Wallet Backups you can read the following:

MultiBit does this automatically in the background using special files that are described in detail in the File descriptions article. These files can often be used to recover your wallet automatically should a problem occur.

In the File Descriptions article you can read the following:

For the purpose of illustration, imagine you have created a new wallet called saving.wallet. Please refer to the relevant help section for your version below (use the 'About MultiBit' menu option if required).

Wallet files for MultiBit version 0.5.13 and later

Here is a list of the files stored with your wallet:

• saving.wallet  =  This is the main wallet file containing your private keys and transactions.
• saving.info  =  This file contains the description of the wallet and the labels you have given to your addresses.
• saving-data  =  A directory containing your wallet backup files.
• saving-data/key-backup  =  A directory containing your automatically generated private key export files.
• saving-data/rolling-backup  =  A directory containing your rolling wallet backups.
• saving-data/wallet-backup  =  A directory containing your encrypted wallet backups.
• saving-data/wallet-unenc-backup  =  A directory containing your unencrypted wallet backups.

Private key backups. In your private key backup directory you will have files called 'saving-YYYYMMDDHHMMSS.key'. Here the YYYYMMDDHHMMSS is a timestamp. With an encrypted wallet every time you add or change the password, add a receiving address or import private keys an encrypted, timestamped export of your private keys is created. This is encrypted with your wallet password. You can use these to recover the essentials of your wallet by importing one of these files into a new wallet.

This should at least put you in the right direction.