EndNote Online (also known as EndNote Web) will be unavailable from 2am BST until 5pm BST (approx) on Sunday 22nd June. This is to allow for scheduled maintenance of the EndNote infrastructure. You can follow the progress of this maintenance on EndNote’s twitter feed, @endnotestatus
The desktop version of EndNote (EndNote X6/X7 etc) will not be affected by this upgrade, but you will not be able to sync desktop libraries with your EndNote Online account during the downtime.