Around 9 a.m. eastern time on Monday, Shawn Van de Vyver, a dentist in Michigan, went to CoinDash's website to check out the project's initial coin offering — a new way that cryptocurrency start-ups are raising money.

Van de Vyver has been building computers and websites for 20 years and started studying bitcoin when it was trading in the single digits (it's now priced at more than $2,000). He also invested a couple thousand dollars in digital currency ethereum, he told CNBC.

CoinDash is sometimes described as the E-Trade for blockchain and Van de Vyver was interested in tracking the project, even if he wasn't yet ready to invest.

Shortly after 9, Van de Vyver got a text from a dentist friend telling him that the site had been hacked.

Visitors to the site had been told to send their ethereum to another address in order to participate in the ICO.

People who followed those instructions had their money stolen, according to the website. Over the course of about a half an hour, more than $7 million was routed to the hacker. According to Etherscan, which tracks the movement of ethereum, some 2,130 transactions took place.