WhatsApp, a messaging service owned by Facebook Inc, said some users were still facing issues while accessing its messaging service.
The problems were primarily centered in the UK and Western Europe, according to DownDetector, a website that provides realtime overview of Internet and mobile services.
"We appear to be having issues again," a WhatsApp spokesman told Reuters.
The company had earlier said it had completely restored the service.
WhatsApp, however, did not provide any details on what led to the outage, which began shortly after 4.30 p.m. UK time on New Year's eve.
(Reporting by Kshitiz Goliya in Bengaluru; Editing by Anil D'Silva)