Website down caused to switch to a WP theme

Yesterday my site was down for a few minutes. I have BlueHost and sometimes this happens.Then it came back and it was ok. Then, I upgraded to the newest version of WP. I had a developer who had to test certain features on my site and showed me a screenshot of my site switched to a WP theme. I didn't do. He didn't do it. How is that possible? So for some time, until I figured out that it was not just a screenshot because the developer switched my theme, it was really my site that had switched to another theme. What can I do to prevent this? I work on my site every few days, but yesterday it was live on WP site with broken links and missing pages. Thanks!