Q: Access problems after not using mac for several months

They probably referred to the FileVault recovery key. Can that be correct? That would mean your husband forgot the password of his Mac. As long as you don't do that, there will be no problem.

Here's some background information for you: Use FileVault to encrypt the startup disk on your Mac - Apple Support

Also see my response to Filevault encryption

Decrypt the old file vault key if you have written in some diary .its a secret key that is meant for the user not be disclosed to apple or any one .if you have deleted that old file vault key and the system is with you not to worry , whenever you feel, file vault key can be set once again .

please state what do you mean by key chain  in a very simple way .

The keychain in Mac OS X is Apple's password management system. A keychain can store all your passwords for applications, servers, and websites, or even sensitive information unrelated to your computer, such as credit card numbers or personal identification numbers (PINs) for bank accounts.

In short: yes. The only way you can lock yourself out from your Mac, is by enabling FileVault, not instructing it to 'leave a key' with Apple and forgetting your own 'key' (password).