If you can't access your cloud storage service, the problem could be any number of things, from your own computer, to the servers in the cloud. To check if the problem is on the service provider's end, check out the system status pages for your provider. Here are the status pages for Dropbox, iCloud, Google Drive, Box, and SkyDrive:
The Dropbox status page provides the status for Dropbox web access, as well as desktop client access. Details of updates are also provided below the status box. To check Dropbox status, go to http://status.dropbox.com.
Apple's iCloud status page provides current details for their services. You can also view details of any recent issues. To check iCloud status, go to http://www.apple.com/support/icloud/systemstatus.
Google's Apps Status Dashboard provides the status of multiple Google services, including Google Drive. When there's an outage or disruption icon, you can click on it for more details. To check Google Drive status, go to http://www.google.com/appsstatus.
The Box (formerly, Box.net) status page breaks down the status of individual functions. You can see the status of downloads, sharing, sync, uploads, and more. To check Box status, go to http://status.box.com.
Microsoft's Live status page lists SkyDrive, as well as Hotmail, Calendar, Messenger and other Live services. To check SkyDrive's status, go to https://status.live.com.
That's it. The next time your cloud storage service goes down, you'll be able to check to see if it's your provider, or if it's a problem with your own system.