Lost notes

Start by checking if your Notes are enabled in iCloud, go to:

Settings → Apple ID → iCloud. Find Notes in the list, if Notes is not enabled, enable it. If Notes IS enabled, choose to disable, restart your iPhone, make sure you're connected to Wi-Fi, then go back and enable again.

Also check that you are looking in the right folder. iPhone can save notes both locally on your iPhone and in iCloud. As you can see in my demonstration below, my Notes app is empty when it is opened. This is because the folder above my local Notes is displayed - and since that folder is empty, no notes are displayed. Tap Folders at the top and select the folder with content. In my case, my notes are in iCloud.

If your notes are neither in iCloud nor Local, or in Recently Deleted, they are unfortunately gone. If you have a Backup from when your Notes were on your iPhone, then you can try to restore from it. But remember, if Notes are synced in iCloud, they won't be saved in an iCloud Backup. If you have a backup made from your PC or Mac, try it.

You can see how to restore from a Backup here:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support