Evernote was resuming operations early Wednesday following a denial of service attack.
The archiving software and services company said its online services had been unavailable and that users may experience problems accessing their accounts.
"We're actively working to neutralize a denial of service attack," the company wrote on Twitter early Wednesday.
DOS attacks bombard a server with traffic, which can slow it down or cause it go offline altogether.
Launched in 2008, Evernote has grown to over 100 million users, a milestone it announced last month. The incident marked the first successful DOS attack on the company in years.
"We're continuing work to mitigate the attack and service has been available again since about 6:15 p.m. PT," Ronda Scott, a spokeswoman for Evernote, wrote in an email.
"We expect that there may be a hiccup here and there in the coming hours, but Evernote is now accessible."
The identity of the hackers is unknown, Scott said, adding that their attack efforts are continuing but the effects have been mitigated.