Facebook has denied it was hacked despite the most severe outage in the firm's history that kept users locked out for more than 14 hours.

Reports began at around 4pm GMT (12pm ET) yesterday that the Facebook 'family' of apps, including Messenger, Instagram and WhatsApp, were down.

Mark Zuckerberg's firm has blamed a 'database overload' on its network of servers for the downtime, rather than an external attack.

Speculation is growing online that the outage was the result of distributed denial-of-service (DDoS) attack, although the firm says this was not the case.

The current problems at Facebook certainly fit this pattern - with the social network, Instagram and WhatsApp also served by the same computer systems.

Experts aren't convinced, however, saying that their internet monitoring systems have not detected any hack attack and that the issue appear to have come from inside the world's largest social network.

Hackers regularly attempt to take big companies like Facebook offline, often through the use of DDoS attacks.

These attacks attempt to crash a website or online service by bombarding them with a torrent of data at exactly the same time

The surge of simple requests overloads the servers, causing them to become overwhelmed and shut down.

Cyber-attackers often use botnets – networks of computers brought under their control through infection using viruses and other malware - to launch these attacks.

This has further fuelled social media users to question whether this is the real reason behind the outage.

Some are concerned that their personal data may have been compromised, a particular worry given the recent privacy scandals that have hit Facebook.

Twitter user Adam Aris Kelly Life Coach said: 'Some reports are saying major hack attack on #FaceBook and #Instagram...Yikes.'

Rachelle added: 'If my data is compromised because of a hack I deserve to know.'

Mister Disc said: 'With the billions Mark Zuckerberg has you'd thing FB would have military grade hack proof servers & best tech people, but no...'

Experts aren't convinced by the hack attack theory, however, and believe Facebook's explanation of a problem with hardware within the company is more plausible.

Speaking to MailOnline, a spokesman for San Francisco-based internet monitoring firm Thousand Eyes, said: 'The cause would appear to be internal rather than a network or Internet delivery issue -for example we saw "500 internal server errors" from Facebook.

'Given the sheer scale and continuous changes that these web scale providers are constantly making to their applications and infrastructure, sometimes things break as a result of these changes, even in the most capable hands.'

'There's no collaborating evidence of any kind to indicate a malicious attack,' added Troy Mursch, a security researcher who runs Bad Packets Report, a site that monitors botnets used by hackers, speaking to Wired.

'In regards to an actual attack or any widespread attack, we can confirm that is not the case there.'

There are a number of explanations as to why problems with Facebook's own hardware could have caused the outage.

The firm's claims of a 'database overload' on its network of servers could be caused by a range of internal complications.

The 500 'internal server error' messages detected by internet network analysts can be prompted to a variety of snags.

With a network of servers - the computers that relay traffic to and from the firm's apps and their users - as large as Facebook, complications are bound to arise.

Planned maintenance of the software databases used to ferry this internet traffic, as well as the hardware they are stored on, can lead to scheduled downtime.

In this case, the outage clearly caught the company by surprise, which would explain why it took them so long to bring their apps back online.

Facebook has so far remained tight-lipped over the exact cause of the 'database overload'.

Facebook and it's related apps creaked back into action today after a 14-hour outage for some users as insiders admitted they were 'racing to spin up new machines as others go down'.

Social media users in parts of the US, Europe, South America and Japan were worst hit by the problems which also affected its apps Instagram and WhatsApp.

The problems resulted in Facebook's most severe outage in its history.

The last time Facebook was disrupted to a similar extent was in 2008, when the website had 150m users - compared to around 2.3billion monthly users today.

A source told NBC Bay Area its database was 'overloaded', adding: 'We are racing to spin up new machines as others go down. Mostly resolved... but it takes time.'

Issues with accessing the website began yesterday – and by this morning most users had their service restored, although some people could still not log in.

Facebook, which gets much its revenue from advertising, was still investigating the overall impact today 'including the possibility of refunds for advertisers'.

A Facebook spokesman tweeted shortly before 6pm last night: 'We're aware that some people are currently having trouble accessing the Facebook family of apps.

'We're focused on working to resolve the issue as soon as possible, but can confirm that the issue is not related to a DDoS attack.'

Daily Mail