Singtel fibre broadband users were unable to access the Internet for more than 12 hours after a service outage.
The problem started at about 8.45am yesterday, and remained unresolved at 11pm.
In a statement issued at about 8pm, the telco said its servers were unable to assign Internet protocol (IP) addresses to its customers' modems. IP codes are numerical addresses that help computers communicate with one another over the Web.
A Singtel spokesman said: "So far, while we have ascertained that the service disruption is not due to a DDoS attack, we are not ruling out other plausible causes.
"We will be working with our vendors throughout the evening and overnight to isolate the problem, and get our broadband services up and running again."
A Distributed Denial of Service or DDoS attack works by getting thousands of infected computers to overwhelm a target site, which then cannot cope with the spike in traffic.
About an hour after the problem started yesterday, Singtel, in a post on its Facebook page, said it was aware of the issue and working on a solution.
The telco continued to post updates throughout the day, and offered to waive charges for mobile data services for affected customers who are also post-paid customers.
Singtel's Facebook page was yesterday flooded with thousands of comments from irate consumers.
Among them was Mr Koh Kang Jun, 18, who said he kept checking Singtel's Facebook page for updates every 30 minutes. Speaking to The Sunday Times, he said: "At about 11.30am, my Internet access just stopped abruptly.
"I'm quite frustrated because I'm a student and an avid gamer. However, I can also empathise with the staff working behind the scenes."
Businessman Rocky Lim, 62, said he was unable to connect to the Wi-Fi network at home, but was still able to surf the Internet using his mobile phone. If he was unable to access the Web on his phone, too, he would have been angry, he said.
Just two weeks ago, Singtel was fined $145,000 for two separate pay-TV disruptions that resulted in customers being tuned out of programmes across more than 10 channels each time.