Yahoo email users across the world were unable to access some services after the site crashed, just after 7am on Thursday morning.
Downdectector said thousands of Yahoo users were complaining of the outage across the UK and US.
Yahoo customer care said in a statement, “You may not be able to access some of our services, including email.
“Our top priority right now is getting this fixed. We appreciate your patience.”
Adding, “We understand how difficult this must be for you. We are working especially hard to fix this. Please stay tuned for updates.”