This usually happens if there is a "fault notes", in your Notes App (in your iPhone).
Don't ask me how it get there, but it happens.
This is what I did and it works. There is no need for phone reset or any restart.
1) Close all applications on your iPhone.
2) Login to icloud.com
3) Click on Notes (You might not be able to see any notes now because one of the "faulty notes" is crashing the app).
4) Just click "Create a new note". Create a new note and leave it there. Sign-out.
5) Go back to your iPhone and start the Notes App again.
6) Wallaaa... you can see it working again, like a charm.