What happens to you if (when) the market crashes?

[Account Closed]

Hey everyone, I recently decided I’m going to build a fourplex to owner occupy. I would potentially walk into this deal at 0 down for a $500,000 fourplex. I have one SFR rental, and was just curious, what happens to me if I have that and the market crashes? I keep YouTubing and looking for information but I don’t really understand why so many people lost so much? Did they invest incorrectly? We’re their mortgages called up? I just want to know what can happen and how to protect myself. Thank you!