On quite a few occasions I’ve had clients send me a complete backup of their site during the discovery period. Usually this is because they don’t have a Release Candidate / Quality Assurance / Staging / Development version(s) of their site where changes can be made, and approved, without affecting the live site.
Without standing on my soapbox too long – if your website lacks a staging site, version control for custom code, or backups – you should address them immediately.
In this case, I was glad the client had at least one backup (the one that was provided to me) which I could turn into a local WordPress installation to begin development. Since the client didn’t provide a WordPress Admin user and password, I just decided to insert myself into WordPress.