SAN FRANCISCO — Robinhood has become a Silicon Valley darling by presenting itself as a tech-savvy alternative to traditional stock trading platforms. But this week, with the financial markets in chaos, the start-up’s technology failed at the most crucial moment.

On Monday and Tuesday, as global markets rebounded after a sell-off last week and then sank again, Robinhood’s trading platform went offline, leaving many customers to watch their portfolios drop in value without being able to do anything about them.

Robinhood has attracted millions of millennials to trade on its platform, largely by eliminating trading fees and making stock trading as easy as ordering food online. This week, many of those same customers said they wanted to get their money out of Robinhood as quickly as possible.

“For me, the moment they get up I am going to try to get out and switch out to someone else,” said Robert Kaufman, 30, an information technology worker in San Diego.