U.S.-based Dyn, a company that monitors Internet infrastructure, said the reason for the outage was not known but could range from technological glitches to a hacking attack. Several U.S. officials close to the investigations of the attack on Sony Pictures said the U.S. government had not taken any cyber action against Pyongyang.
U.S. President Barack Obama had vowed on Friday to respond to the major cyberattack, which he blamed on North Korea, "in a place and time and manner that we choose."
Dyn said North Korea's Internet links were unstable on Monday and the country later went completely offline. Links were restored at 0146 GMT on Tuesday, and the possibilities for the outage could be attacks by individuals, a hardware failure, or even that it was done by North Korea itself, experts said.
Matthew Prince, CEO of U.S.-based CloudFlare which protects websites from web-based attacks, said the fact that North Korea's Internet was back up "is pretty good evidence that the outage wasn't caused by a state-sponsored attack, otherwise it'd likely still be down for the count".
Almost all of North Korea's Internet links and traffic pass through China and it dismissed any suggestion that it was involved as "irresponsible".
Meanwhile, South Korea, which remains technically at war with the North, said it could not rule out the involvement of its isolated neighbour in a cyberattack on its nuclear power plant operator. It said only non-critical data was stolen and operations were not at risk, but had asked for U.S. help in investigating.
South Korean President Park Geun-hye said on Tuesday the leak of data from the nuclear operator was a "grave situation" that was unacceptable as a matter of national security, but she did not mention any involvement of North Korea.
North Korea is one of the least-connected nations in the world, and the effects of the Internet outage would have been minimal.
Very few of its 24 million people have access to the Internet. However, major websites, including those of the KCNA state news agency, the main Rodong Sinmun newspaper and the main external public relations company went down for hours.
"North Korea has significantly less Internet to lose, compared to other countries with similar populations: Yemen (47 networks), Afghanistan (370 networks), or Taiwan (5,030 networks)," Dyn Research said in a report.