If you're facing issues with iCloud, you're not the only one, as several users on Twitter and Reddit are reporting that they are unable to access the service.
The issue seems to be infrastructure-related, with users encountering "server time out" errors after entering their credentials. IsItDownRightNow shows that the service is currently suffering an outage. Apple's own support system is also listing that iCloud-based services — including iCloud Account & Sign In, iCloud Backup, iCloud Drive, iCloud Keychain, iCloud Mail, iMovie Theater, Photos and iWork for iCloud beta — may be slow for some users.
Update: Apple is now saying that the iCloud issues have been resolved. Overall, the company says that about 40% of all iCloud users were affected by the disruption.