My boss just visited me. His son is back for the holidays. He's been working in Asia for the last few years. He has a ton of photos. He had a ton of photos on his MacBook Air. Last night he upgraded his OS, and it asked questions and he blindly went okay, okay, okay. An hour later he had a fresh install and no photos. My boss asked if anything could be done. Sure, I said, restore from backup. Oops. He has no backups. It's all gone.
This happens a lot. Backup your data.