Notes saved in iCloud may be recoverable - but first you’ll need to resolve the problem of the missing Notes App.
If the Notes App was simply deleted from your iPad, the App can be re-installed from the Apple App Store. Using the App Store App, just use the Search tool - and search for Apple Notes. Once located, simply install as any other App.
Next, assuming that the missing notes were synchronised with iCloud, you should be able to recover any deleted notes within 30-days of deletion. You need to use a web browser to sign-in to iCloud:
https://icloud.apple.com
The procedure to recover deleted notes can be found here:
Delete and recover notes on iCloud.com - Apple Support
If your notes were not synchronised with iCloud, your only hope of recovery is to restore the data from a pre-existing iCloud or iTunes backup. If you don’t have a backup, your deleted notes have been irretrievably lost. More information about restoring from a backup can be found here:
Restore your iPhone, iPad or iPod touch from a backup – Apple Support (UK)