Unless you moved to the United States a very, very long time ago, you did not actually have to give up your citizenship to live in the US. The United States doesn’t acknowledge other citizenships but does not currently force people to give up their citizenship. Canada forced people in some circumstances to give up their citizenship until 1977, however.
You likely just need to apply for a Canadian Passport.
However, if you actually did renounce your citizenship by signing something and submitting it to the government of Canada, you will have to claim your citizenship as a lost Canadian. Immigroup can help you with that. Call us at 1-866-760-2623.