If you performed the work, they were satisfied with it, and you were paid then that's it. Any contract, written or implied, has been filled. There's been accord and satisfaction. You owe them nothing.
Unless you specifically state somewhere that you provide ongoing backups for all client files, or are being paid to maintain such an archive, it's not your responsibility to maintain any archive for clients. The client should have backed up files when they were originally sent.
So..
- Tell them you no longer have the files.
- Tell them you no longer have the files. But you can recreate everything at your current rates (not 6 year old rates).
- Tell them you have files, but there's a fee for the time it takes to pull them from archives, and ensure they are in proper form for delivery today given possible technological advances. I'd also point out to the client that even if files are retrieved from an archive that does not mean they will still be in good, usable condition (files can become corrupt).
- Just give them the files.
- Ignore them entirely.
In broad terms, once a contract has been fulfilled, you are not legally obligated to do anything for a client. Not even obligated to reply to their email.
With all the above written, as a business you really do want to have an archive of past work. If some past client asked me for files 6 years old, I know I have the files - triple redundant backup of all client work since the inception of my business. However, my knowing I have the files may or may not change my response to the client. The archive for my business is not the client's archive. The client should have their own business archive. Failure of the client to maintain their own business archive never means I'm obligated to accommodate any request for files from my business archive.