My notes have disappeared from my ipad

Quit the notes app completely and reboot the iPad. Go to the home screen first by tapping the home button. Double tap the home button and the recents tray will appear with all of your recent apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus ➖ sign in the upper left corner of the app that you want to close. Tap the home button or anywhere above the task bar.

Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.

See if that brings them back. If it doesn't. Backup the phone and restore the iPad from the phone backup. But that will restore all of the contents of the phone backup onto the iPad.

Or look at something like this to extract the contents of the phone.