Apple chief Tim Cook sure sounded like he was spooked on Monday.

After a steep plunge in China’s stock market and as Apple’s stock prepared to nose-dive ahead of the opening of U.S. markets, he took the unusual step of sending an email to Jim Cramer, host of CNBC’s “Mad Money,” saying that Apple was going to be all right.

“I can tell you that we have continued to experience strong growth for our business in China through July and August,” Cook wrote, noting that he gets daily updates on Apple’s business in China.

Cook’s brief letter, which was shared on Twitter by Cramer’s CNBC colleague Carl Quintanilla and then read on-air, appeared to calm market jitters, even as it raised eyebrows. As Cook’s note was being shared widely, Apple’s stock — which initially plummeted as much as 13 percent — made up most of those losses and ended down 2.6 percent. The reversal helped lift the rest of U.S. markets as well. The wild gyrations of Apple and U.S. markets highlighted a deepening concern that the tech sector may be seeing the end of a years-long boom. More than in other industries, the sky-high stocks of such sector stars as Apple, Facebook and Google are built on the assumption of future growth and the ability to exponentially expand sales of gadgets or the acquisition of new subscribers outside the United States.

For a while, the potential for growth looked spectacular, driving share prices up. But any hint that global sales of smartphones or other services may fall short could make those stocks fall all the harder.

Shares of leading tech companies were among the hardest hit early on Monday, with Facebook down as much as 13 percent and Google down 4 percent at one point. The tech-heavy Nasdaq index was down as much as 5.1 percent before closing down 3.8 percent at a 10-month low of 4,526.25.

“The tech industry is a barometer of economic progress and innovation. Therefore, its success is an indicator of general economic progress, which eventually translates into growth,” said Cristian Tiu, a professor of business at the University of Buffalo’s School of Management.

But what’s really terrifying executives and investors in the technology sector is the chilling effect a slide in global markets could have on the many richly funded startups that aren’t trading yet on exchanges.

With so much uncertainty and market volatility, Silicon Valley firms could postpone initial public offerings, cooling a white-hot market for venture funding that has fueled the most lucrative environment for startups in history.

The big question is what will happen to the hundreds of start-ups – a record 131 are valued at more than $1 billion – that are all dressed up for IPOs but could have no place to go.

Among the most anticipated startups are Uber, valued around $51 billion; AirBnB, valued at $25 billion; and Snapchat, valued at $16 billion.