Why my NOTES is gone? Is that possible to reinstall?k Thank

Device or computer?

IOS

Apps - Remove built-in Apple apps from the Home screen on your iOS device or Apple Watch

If necessary, do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Restore - If you see an error when you update or restore your iPhone, iPad, or iPod

Backup - Restore your device from an iCloud or iTunes backup.      

OS

Do you have a Time Machine backup you can restore from?

Do a backup, preferable 2 separate ones on 2 drives. Boot to the Recovery Volume (command - R on a restart). Run Disk Utility and select First Aid. Then re-install the OS.

Reinstall from macOS Recovery.    

OS X Recovery.