The problem was "the direct result of a major regional power failure in Northern Virginia," said Nicolas Graham, a spokesman for AOL.
Service has been restored, he said. However, users were still experiencing problems. AOL IM subscribers said that they could not reach their "buddy lists," of contacts, and others were unable to use the communication tool at all as people flooded the service trying to log back on.
On Tuesday, an AOL spokesman declined to say how many accounts had been affected, declining also to elaborate on the problem beyond describing it as an "equipment glitch."
A switch failure on March 27 also brought down AOL Instant Messenger for a short while. A spokesman Tuesday said that day's outage was not related to what happened in March, when AOL IM users with older versions of the free software continued to have problems for days afterward.
A company source speculated on Tuesday that unusual solar flare activity could have caused the disruption that day. On Monday, scientists recorded intense activity in the sun's 11-year solar flare cycle, and said the flares might affect radio transmitters and, in rare cases, ground equipment. But while there were two major flares on Tuesday, scientists from the National Oceanic and Atmospheric Administration (NOAA) Space Environment Center, which forecasts solar weather, think any problem AOL had with instant messaging that day was likely related to larger telecommunications problems rather than solar flares, said Barbara McGehan, a public affairs officer at NOAA.
Some parts of the service remained available on Thursday while users experienced problems with specific areas like buddy lists, which seems to suggest the problem was an internal network issue, suggested John Deep, a spokesman for Aimster, a file-sharing piggyback application for AOL Instant Messenger.
"If you can log on but can't get your buddy list, that means some of their servers are working and others aren't," Deep said.
That even a short outage was so quickly noticed by many users speaks to the depth of the service's market penetration. AOL claims its 25 million instant messaging users send 656 million messages daily. AOL has had a nearly prohibitive lead for years over rivals Microsoft, AT&T and Yahoo.
Recent figures from Media Metrix indicate MSN Messenger has caught up to AOL in worldwide instant messaging subscribers with 29.5 million unique users in February, although the study was commissioned by Microsoft.