1. If You Have an iTunes or iCloud Backup
1.1. Check iPhone Storage
If there is not enough storage on your iPhone, this problem might appear. Go to Settings > General > Storage & iCloud Usage. If there little available storage on your iPhone, just click Manage Storage and deleted some unwanted apps. Once you have done that, please restart your iPhone.
1.2. Force Restart iPhone
Sometimes the iMessages/Messages are not really cleared. It just gets some glitches after iOS 11.3 update, so that the iMessages/Messages cannot be displayed. This can be fixed by force restarting iPhone.
For iPhone 6 and earlier versions, just press 'Power' and 'Home' buttons at the same time until Apple logo shows up on screen; for iPhone 7 and 7 plus, press 'Volume Down' instead of 'Home' button; for iPhone 8 and iPhone X, quickly press 'Volume Up' button and then 'Volume Down' button, then keep pressing 'Power' button until you see the Apple logo.
1.3. Get Back the Messages from iCloud or iTunes Backup
If force restarting doesn't work and all iMessages/Messages are still missing, you can try restoring your iPhone from old iTunes or iCloud backup. Here are the steps to restore iPhone from iTunes:
Launch iTunes on your computer and connect your iPhone using a lightning cable.
Once iTunes detects your iPhone, click on it and go to the 'Summary' tab.
Click on the 'Restore Backup' option and select the desired backup file from the list appearing on your screen.
Now, push the 'Restore' button to initiate the process.
If you want to restore from iCloud backup, please refer to the guide here.
3.2. Factory Restore iPhone with iTunes
When iMessages/Messages are randomly deleted after upgrading to iOS 11.3, here is another option you can try to fix the issue. It is factory restoring your iPhone with iTunes. Let's see how to do it.
Launch iTunes on your computer and get your iPhone connected through a USB.
Once iTunes detects your iPhone, click on the device icon and tap the 'Summary' option.
Click 'Restore iPhone' and then tap 'Restore' to confirm.
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Any app suggestions or topic recommendations are appreciated in the comments down below.
