iCloud is a great place to store backups for your Apple devices. You can have your iPhone or iPad set to automatically back up to iCloud, then you can forget about it. It will do a backup every evening when your device is plugged in to charge.
There may come a time, however, when you notice your backups have stopped working. No matter what you try, it just won’t start. For iPhone, in particular, this seems to be a common problem.
In your iPhone Settings, tap iCloud, then Storage & Backup. Make sure iCloud Backup is turned ON, then tap the button to Back Up Now. If it seems to hang up on estimating time remaining, then finally leaves the message "The last backup could not be completed," try one or all of the suggestions below to get things moving again.
1. Do a soft reset on your phone and try again. Hold down the Home button (round button below screen) and your Power button at the same time. Slide button to power off, then after you turn it back on, go to Settings and re-try backup.
2. Delete iCloud account, then sign back in. In iCloud Settings on your iPhone, scroll down and tap Delete Account on the lower portion of the screen. You will receive a couple warnings letting you know it will remove your documents stored in iCloud, then it will ask if you want to keep them on the phone or not. Tap Delete and select to keep the files. Sign back in with your Apple ID and password, then try backup again. (Some have also suggested to sign in with another Apple ID or create a new one, then if that works, try your old one again.)
3. Turn off storage for Camera Roll. This is the one that worked for us. First, in iCloud Settings / Storage & Backup, you can delete your last backup to free up some space.* Then while still in Storage & Backup and Manage Storage, tap on your iPhone under Backups. On the screen that follows, look beneath Backup Options and Next Backup Size, then turn Camera Roll to OFF. Repeat steps to back up your phone.
* Optional: To delete your last backup, follow the same steps above and tap on your iPhone in Manage Storage. Scroll to bottom of screen and Delete Backup.
Hopefully, one of these three tips will help your iCloud Backups to start working again. If not, you still have the option of backing it up to your computer or you can contact Apple for more instructions.