As a phone, the most important data on the iPhone must be the contacts. It would be a disaster if you all your contacts just vanish without any trace. Phrases about how to recover lost contacts persistently appear in online discussions which does raises a few eyebrows. What causes countless iPhone users to lose contacts so suddenly and so unexpectedly? There is no single reason can explain for various kinds of contacts loss. This post is going to talk about a couple of possibilities why contacts get lost.
Reason 1: iOS version updating
After updating to iOS 8.4 for example, people may find out that contacts or other kinds of data like notes or messages etc disappear and there is no way to reclaim the lost data unless you have backed up with iTunes or iCloud, for contacts lost during the updating process had been overwritten and erased permanently.
Reason 2: iPhone Jailbreak
Although not a direct cause, having your iPhone Jailbroken could lead to situations in which iPhone contacts are lost. Jailbreak is known for the vast opportunities it provides to iPhone users to modify and tweak their iOS devices. It goes without saying that while some tweaks are fun and useful, others could bring unexpected results to the iPhone and its data.
Reason 3: iPhone Reboot
An ordinary reboot of your phone -- which Apple calls a restart -- does not cause you to lose data, except for any unsaved files you have open in apps without autosave. If your phone freezes and won't display the power-off slider, you can reset it, which is a forced reboot. This causes you to lose unsaved data in running apps, even if those apps would normally save automatically when closed.
Reason 4: iPhone Restoring
If you are trying to restore your iPhone with the iTunes backup file, some of the newly added contacts or any other information would be lost because of overwriting and erasing.
Conclusively, backing up your iPhone regularly is the only way to protect your data in case of data loss, including the contacts loss. Provided that you enabled automatic syncing your iPhone with iTunes on your computer, it automatically makes a backup. You can alternatively enable iCloud backups in the "Settings" app by tapping "iCloud" then "Storage & Backup" and switching "iCloud Backup" to "On." After setting this option, the phone automatically backs up when it is connected to power and Wi-Fi and is not in use.