Hello and welcome to Apple Support Communities xenia2606,
Thank you for reaching out to us regarding your iCloud notes. We definitely want to help you with this issue. Did something specific happen prior to the day your notes disappeared? Hopefully you do have an iCloud backup of your data, so we should be able to restore from a backup. But before we do that, we want to double check where they might be. Is it possible they are included with a 3rd party email, such as gmail or yahoo? Were any of the notes under the "On My iPhone" option? Can you see them at iCloud.com? Do you have enough storage or do you need to get more? Check out the troubleshooting steps below to determine if one of the options enables you to find your notes.
If you're missing notes on your iPhone, iPad, or iPod touch
View notes and locked notes on iCloud.com
Change your Notes settings on iPhone
Manage your iCloud storage
Make sure your email passwords are entered as well, in case the notes are under the email provider.
If none of the above options enable you to find your notes, we would suggest erasing your iPhone and restoring from the backup prior to your notes were missing.
Restore your iPhone, iPad, or iPod touch from a backup
Thank you for contacting Apple Support Communities.