Facebook users have reported being locked out of their accounts for failing to respond to a message some mistook for spam.
Users who have realised the warning is real have tried to follow the social network's instructions - but an issue is currently preventing them from regaining access.
A notice was sent to selected accounts deemed at particular risk from hackers, telling them to enable an enhanced security feature called Facebook Protect.
Many users were given a March 17 deadline to set it up themselves or face having their account locked until its done.
However, some Facebook users thought the message was a scam and ignored it as the email address used, security@facebookmail.com, didn't appear legitimate to some.
When the deadline passed, however, users were told they had to switch on Facebook Protect to get in but those that went ahead with it claim the set up system is broken meaning they are still locked out.
And the authentication tools - such as a text message to prove its you - are not working.
"I got locked out from Facebook indefinitely today because I didn’t respond to emails from Facebook (that looked like a scam) about its new Facebook Protect system, which I was required to enable by today," one user said.
"So far, the text and security key options don’t work, many report."
Another user said: "This Facebook Protect thing is so annoying because it’s not letting me turn it on and I need Facebook for work so I’m really hoping Facebook fixes the stupid code."
Facebook Protect is a special monitoring programme for people at heightened risk of being targeted by hackers.
This includes those working in human rights, journalists and Government officials.
