If you didn't backup it before you can't able to backup today if it's already replaced with a new one.

Have you tried to search for it on your hard drive may be the old one still exists on your hard drive?

Use some tools like "Everything" where you can easily search a file without long loading.

Use keywords like .key or wallet or mbhd.wallet.aes for multibit HD

Once you found the old one make a original backup copy before you extract your all private keys.