Fidelity said it resolved a technical issue that had blocked clients from accessing their online accounts for part of Wednesday. The issue had affected all online accounts.

The fund giant announced in a service note on its website in the early afternoon that "the issue has been resolved."

Earlier in the day, Fidelity spokesman Mike Aalto told CNBC that "our site's down, but I can tell you it's an internal issue. Clients aren't able to log in to their accounts right now." He said that individuals could still make trades by phone.

Aalto said the disruption was not the result of any malicious activity or hack.

Fidelity is the third-largest brokerage as measured by client assets, which totaled $1.7 trillion at the end of 2016, according to Bill Butterfield, a researcher at Aite Group. He estimates that Fidelity accounts for 8 percent of total U.S. wealth management.

Fidelity also manages retirement and mutual funds, with total assets under management of $2.2 trillion and 67 million customer accounts.

The risk for companies experiencing technical glitches is turning customers away. "If this issue persists or is later attributed to a hacker, then that may boost client defections, but that remains to be seen at this point," Butterfield said. "The amount of business lost directly because of this will likely be minor if the issue is resolved quickly."