5 Years Inadmissible to Canada- Ouch!
The second you realize you made an innocent mistake answering a question on your application retain experienced Immigration Counsel. When you forgot to include that you had been refused a visa to the USA Canadian Immigration will know and imply you misrepresented a material fact. Accept you made a mistake and have someone apologize for you.
Misrepresentation
https://laws-lois.justice.gc.ca/eng/acts/i-2.5/section-40.html
Roy Kellogg
cvimmigration.com