Update: March 27, 2020
A little light in times of the corona crisis: Canada opens borders for a select group foreign nationals
As of today, Canada allows entry of temporary foreign workers, international students who received the approval of their study permit before 18 March and people who were approved for permanent residence before 16 March, but who had not yet travelled to Canada. People who travel to Canada by air need to pass a health check before they are allowed to board their flight. If you are currently in the United States, you can travel to Canada by land. Anyone who shows symptoms of COVID-19 (fever, coughing or difficulties with breathing) will not be allowed to enter Canada by air. Upon arrival in Canada, your health will be assessed again. Afterwards, you must isolate for 14 days. Also if you don’t have symptoms, quarantine is mandatory. Only people providing essential services and truck drives who regularly cross the border to maintain the flow of goods might be exempt from the isolation requirements.
Update: March 17, 2020
Canadian government closing the borders
Prime Minister Justin Trudeau announced that the Canadian Federal Government closes its borders to most foreign travellers, with the exception of Canadian citizens and permanent residents. These exemptions also apply to immediate family members of Canadian citizens, diplomats, air crews and U.S. citizens.
The travel restrictions will be in force from of March 18 at 12.01 a.m. EDT. This might also have consequenses for you if you recently received a visa approval, for example within the IEC-program. From of March 18, access will be denied to tourists, travellers with a temporary work or study permit. People who want to access Canada on a temporary status (work- or study permit) will not be permitted to enter Canada to activate their visa.
Source: https://www.canada.ca/en/immigration-refugees-citizenship/services/coronavirus-special-measures.html