Solution 1: How to Restore iPhone Calendar from iCloud Backup
If you previously synced your iPhone calendar with iCloud, follow these:
# 1: Be sure iPhone is connected with Wi-Fi.
# 2: Tick “Restore From iCloud Backup” option on your iPhone setup.
# 3: Enter account and password to login iCloud.
# 4: Choose your backup need to retrieve calendar event and restore to your iPhone.
The whole process cost a large amount of electricity so you must plugged iPhone into the charger at first.
Solution 2: How to recover calendar from iPhone without backup
We are not ruling out the situation of no iCloud or iTunes backup. In this case, iPhone Data Recovery program is the smart way to recover deleted calendar from iPhone without backup. Tenorshare iPhone Data Recovery program allows you to find lost calendar event on iPhone 6, 6 Plus, 5S, 5C, 5, 4S, 4 and 3GS directly with 3 simple steps. Moreover, text messages, contacts, photos, notes, call history, WhatsApp messages and other files also are able to restore by the same token.
See the full instruction:
Step 1: Click here to download and install Tenorshare iPhone Data Recovery tool and run it. Stay in the recovery mode of “Recover Data from iOS Device” and connect your iPhone with computer.
For iPhone 5S/5C/5/4S, click “Start Scan” button to scan files from phone.
For iPhone 4/3GS, get into DFU mode to scan lost data by following the steps below:
>>> Hold Home and Power button at the same time for 5 seconds. If your iPhone has been powered off already, click "Start" button to enter DFU directly.
>>> Keep holding Home and Power button until the device screen turns to black.
>>> Release Power button and keep holding Home for 10 seconds.
After enter DFU mode successfully, the tool will automatically scan your iPhone
Step 3: All lost files will be found and displayed in list after the scan. Check “Calendar” catalogue on the left to preview lost calendar entries. Mark the calendar events need to restore one by one or just click “Check All” if you have such a need, then click “Recover” button to save the calendar on your computer.
Similarly, restore iPhone calendar from iTunes backup by means of the second recovery “Recover Data from iTunes Backup File”.
iPhone icon missing for no reason or you thought you accidentally deleted it. Actually, it is impossible to delete built-in apps on iPhone.
Generally, reset Home screen layout, your disappeared calendar icon will show up again.
Follow the instructions: Settings > General > Reset > Reset Home Screen Layout.
If that doesn't work, try to restart your iPhone.
Follow these steps: Hold down the Sleep/Wake button, then wait for the Slide to power off message to appear> Slide to turn off> wait a couple of seconds and then press and hold the Sleep/Wake button again until the Apple logo appears.