British Airways has said its IT systems are "back up and running" following hours of chaos which resulted in the cancellation of dozens of flights - but there could still be "intermittent issues".
The airline axed more than 50 departures and more than 20 arrivals on Thursday, with some Friday passengers also being warned of delays and cancellations.
On Twitter just before 11.30pm, BA said its systems were "back up and running", although there "might still be intermittent issues".
Earlier it had said it was "aware of a technical issue, which we have been working hard to fix", a line it was repeating in response to queries in the early hours of Friday.
A spokesperson from Heathrow Airport said the cancellations are not related to a three-day strike by security guards at the airport over pay.
Sorina Anastasiu - a 33-year-old recruiter living in London - was due to get married in her native Romania in June, but she won't make it.
The law there requires couples to have their marriage rubber stamped 14 days before the wedding, so she was flying home to complete the forms.
She told Sky News: "You have to complete these forms 14 days before and I definitely can't do that now. So I've missed my wedding, basically. I had a bit of a breakdown about it earlier."
"It's been a really, really tough day. I've been here since three o'clock so it's been five hours now. I was supposed to fly to Romania because I'm getting married and needed to get paperwork done.
"Our appointment was tomorrow. Now, I can't get a flight until Saturday. The worst thing is that I've already checked in and they asked me to check my hand luggage, which has my ceremony dress inside it."
She added: "Nobody can get their checked luggage back and there's nobody to help. There's just one landline phone in arrivals marked 'luggage enquiries' and that's the only way of contacting the baggage crew.
"All I have on me is my phone and my wallet. It's insanely frustrating.
"It's very poorly managed. I mean, my situation isn't life or death, but there are elderly people here, people who hardly speak English…There are people here who've got their medication in their bags and they can't access them.
"The staff are saying they'll reimburse us for hotels, but people have nothing on them - no clothes, nothing, so they don't want to leave. It's heartbreaking to see these people struggling in the airport with nobody to help them."
Callum Thomas Graham, a head chef, vented his frustration on Twitter after his flight was cancelled at short notice, disrupting his travel plans.
In a message to BA, he wrote: "How can you send out an email to someone cancelling a flight 16 mins after your offices close.
"Leaving no options apart from cancel my booking absolute disgrace."
The airline responded: "We're really sorry that this has happened, Callum.
"There were some network issues yesterday that affected a lot of the schedule."
Passenger Phil Renshaw arrived at Heathrow only to find his connecting flight to Manchester cancelled and rebooked to the following day.
He tweeted that BA customers services at the airport was "absolute chaos" with a long queue of people waiting for help.
James Johnstone wrote: "Flight cancelled at 11 hours notice??? No explanation. Impossible to get though on phone. Three years of planning a project out the window. What the hell is going on????"