Evernote was hit by a denial of service attack and resumed operations early Wednesday.
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.
The identity of the hackers is unknown, the company said, adding that their attack efforts are continuing but the effects have been mitigated.