I would credit the client, redo the work, and own up to the issue. I would also set something in motion to prevent it from ever happening again. Be it an email alert of files to be deleted a day prior. A calendar reminder for myself to check the contents of the shared storage space. Something like that. It's a great opportunity to show your client how valuable they are to you, and let them know that despite your technical skill - you're human like everyone else. I'm sure they've had something like that happen in some way shape or form...