If you are a Canadian citizen within the first year of your citizenship over the age of 18 and you've lost your citizenship certificate, you must contact Immigration, Refugees and Citizenship Canada and submit an Application for a Search of Citizenship Records to obtain a new one. Follow this link for more information: http://www.cic.gc.ca/english/information/applications/search.asp
If you have lost your PR card, please see the instructions here https://www.canada.ca/en/immigration-refugees-citizenship/services/new-immigrants/pr-card/apply-renew-replace.html