The firm said it was hit by a distributed denial of service attack that began on Tuesday.
Some members were temporarily unable to synchronise their filings from one device to another while it continued.
The California-based company announced last month that it had more than 100 million users.
Distributed denial of service (DDoS) attacks are caused by what can be thousands of computers sending huge amounts of data to a target's servers in an effort to overwhelm them.
This sometimes involves hijacked PCs - whose owners may be unaware of their involvement - in what is known as a botnet.Evernote says it has restored its cloud-based note synching service
This is not the first time the storage service has been compromised. In 2013 it said hackers had managed to access user names, email addresses and encrypted passwords.
However, it appears that the latest cyber-assault is more limited.
Spokeswoman Ronda Scott told the BBC that the cyber-assault, caused by an unknown perpetrator, began at 14:25 local time [22:25 BST] on Tuesday and had not yet ended.
"We continue to mitigate the effects of the attack, but have successfully returned Evernote to service," she added.
"As is the nature of DDoS attacks, there was no data loss, and no accounts were compromised."