In an emailed statement to CNBC, an HSBC spokesperson explained that the outage was due to a "denial of service attack" but that the bank effectively raised its defenses.
"Customer transactions were not affected," HSBC said.
A denial of service attack usually consists of hackers flooding a network with traffic, which then overwhelms and takes websites offline.
The banks says services have been restored and are operating as normal.
"We apologize for any inconvenience this incident may have caused," HSBC said.
It's the second time this month that users were unable to access accounts online, but the bank has not blamed the outage reported at the beginning of January on a cyberattack, with the company's Twitter account citing an "internal technical issue."