Stansted Airport was ground to a halt after a power failure caused mass delays on Sunday afternoon.
Planes were delayed and shuttles canceled to and from the gates with travellers complaining of being stuck on trains for up to 45 minutes.
Originally it was thought the airport railway station was evacuated after its fire alarms went off.
But an airport spokesman confirmed that a "power outage" affected the whole site and systems had to be rebooted.
Although the power outage has been resolved, all of the airport's systems have been affected.
A spokesman for Stansted said: "The track transit system taking people to and from gates is still not functioning, and we're waiting for that to start working.
"There is a coaching operation in place, and the issue has been resolved but it's had a knock-on effect. It's a busy time of year and busy time of day, so this is going to roll on across the day potentially. There was a power outage earlier on that affected all of the systems.
"It needed to be restored through various systems across the whole airport. Our focus now is restoring power to all of those areas that have been affected by that."
Rail operator Greater Anglia posted on Twitter to say a "controlled evacuation" was underway and trains were unable to run to or from the airport.
They added in a statement: "Earlier today, we experienced a brief power outage in our terminal and train station.
"Power has been restored and we are working hard to get all systems up and running as quickly as possible, but we are experiencing some ongoing disruption and apologise to those affected.
"Passengers are advised to check the status of their flights with their airlines.
"Those due to travel by train to or from the airport should also check whether their service is operating before they travel.
"As we recover all operations, safety is our top priority and we thank passengers for their understanding."
Frustrated passengers took to social media to complain about the chaos.
Emma Aldous said: "What is going on? Waiting 15 mins for the shuttle at Stansted Airport no communication."
Another person wrote on Twitter: "Power cut at Stansted airport. The queue for Ryan Air check-in is enormous, and growing."
And Nick de Bois said: "Absolute chaos for hundreds at Bishop’s Stortford station as apparent power disruption caused the cancellation of trains to Stansted Airport.
"Some amazingly good-tempered and patient visitors trying to get home that I spoke with. More buses needed if poss and very few taxis available."
