Ok I'll try and break this down as simply as possible.
1. I created a site for my client on my own site/server
2. Once the site was done and approved, I transferred all files to the client's site/server
3. I fixed all links so site was running independently on client's site
4. I removed all files from my site
5. *Months pass without incident*
6. I removed old WordPress installs on my server that are no longer in use, one being the one for this client
7. Their site goes down, Database issue in wp-config.php
8. Fixed DB_NAME, DB_USER, _DB_HOST to match client's MySQL DB
9. The site has "reset" to the blank theme, as if it was just installed, even though I can see all my images and other files on the FTP.
10. Lay down, cry... a lot. Help please.