What’s the worst that could happen if you invest in the S&P500?

I'm super new to investing and I'm thinking about putting 100 eur savings per month in the S&P500. Since the S&P500 consists of the 500 largest companies in the US, its fairly low risk if something happens to one of the companies.

But what's the doom scenario here? What's the absolute worst that can happen to your S&P500 investment?