The iCloud is the heart of the Apple ecosystem and users rely on it for a wide variety of services, including iCloud Backup, iCloud Drive, Notes. Find My iPhone. When the iCloud services go down users won't be able to access these features so it's fortunate it doesn't happen very often.
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It did happen overnight last night and this morning however as several users reported not being able to access these critical services. Apple posted an update on its System status page and noted that iCloud was being affected by a service disruption although this wouldn't hit every user.
Apple do not a run a public facing Twitter account for the iCloud so they were slow to acknowledge the outage on any of their public channels, despite multiple reports of slowdowns. Fortunately this doesn't happen very often, and if you'd like to check if there are any current issues with the iCloud service, you can check on its status here.