I accidentally deleted my notes app. How can I get it back?

See this support document. .Remove built-in Apple apps from the Home screen on your iOS device with iOS 10 - Apple Support

While it begins with how to remove them, if you scroll down further, you will see how to get them back.

Check by logging into www.icloud.com with your ID and see if your notes are there. If they are there, just turn off sync on the iPhone, and then turn it back on again, they should sync back.

lisad8 wrote:

what if they are not showing in icloud either??

If your Notes are not in iCloud, then you were only saving them on the phone itself. In that case, the only place they would be is in the iPhone backup, unless you were syncing them with other account, such as an email account, such as Gmail, or other email provider that handles notes. However, only you would know that. If you have a backup of the device that was made while the notes were on the phone, you could restore to that backup. However, understand that anything done since that backup would be lost.

My app was removed after updating to ios11, I’ve restored it but all the notes I had have gone. How do I find them again? They were backed up to iCloud

what if they are not showing in icloud either??