As a possible example to why it changed perhaps something went wrong at your host so they put a backup back on. Though they should have told you and it shouldnt have been that old. So who knows.
They've been pissing me off in general my hosts actually.
I wanted to use SVG on my site and it wasn't working. Turns out they didn't have the MIME type correctly set up. I asked them to fix it and after a long winded discussion we ended up at them saying that the engineers were unable to change MIME types.
How is your database? I hope it's just your settings.php file that they've messed up. If your data is gone, I'd demand money back from your hosting company.