From NBC:
Flights across the United States resumed Wednesday morning, several hours after the Federal Aviation Administration suffered a computer outage that forced it to halt all departures nationwide while it scrambled to resolve the issue.
The FAA said the crippling delays that affected thousands of flights appear to have been caused by a problem in the Notice to Air Missions (NOTAM) system, which sends pilots vital information they need to fly.
Investigators have seen no evidence of a cyberattack, White House press secretary Karine Jean-Pierre said.
From AP:
Before commencing a flight, pilots are required to consult NOTAMs, which list potential adverse impacts on flights, from runway construction to the potential for icing. The NOTAM system broke down late Tuesday, leading to more than 1,100 flight cancelations and 7,700 delayed flights by midday Wednesday, according to the flight tracking website FlightAware. The chaos is expected to grow as backups compound. More than 21,000 flights were scheduled to take off in the U.S. Wednesday, mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium. Airports in Chicago, Los Angeles, New York and Atlanta were seeing between 30% and 40 of flights delayed.