Accidentally Tap "Don't Trust" on iPhone/iPad/iPod touch. How to Undo it?

There is a safe and trusted mechanism of iOS 7 or later - when you plug iOS device to your computer, the device will pop up an alert "Trust This Computer?" The majority of users will choose to trust. Unfortunately, if you accidentally tap “Don’t Trust”, what would you to undo it?

1. Disconnect and connect iOS device to computer again

The first thing you can try is to simply disconnect iDevice from the USB port of the computer, waiting for about 5 to 10 seconds and then reconnect iOS device to PC. This should trigger the alert of “Trust This Computer?” on iPhone, iPad or iPod touch again. Then make sure you tap "Trust".

2. Reset dialog warnings in iTunes
If method 1 doesn’t work, you can go to "Preferences" in iTunes to reset all dialog warnings, which will force the alert to appear again on the computer, rather than on your iOS device, so that you can start over.
Step 1: Disconnect iOS device.
Step 2: In iTunes, go to Preferences > Advanced. Then click “Reset warnings”.

3. Reset Privacy settings on iOS device
Reset your Privacy settings. Settings > General > Reset > Reset Location & Privacy. Then connect device to computer again. The alert will appear.

Reference: https://support.apple.com/en-us/HT202778