In a historic united front to the Coronavirus pandemic, the United States and Canada agreed to close their borders to non-essential travel. Tourism is not considered essential travel. Only citizens and residents repatriating, diplomats, and other approved persons may cross the border.
You may still be able to book a flight from or to Canada from the USA, but be aware that the airline is now required to screen you and your reason for travel before allowing you on the plane. Of course, on landing you will be required to see an immigration officer to prove your travel is indeed essential.
As for driving across the border, this is not allowed for tourists either. Regular trade is considered essential, to ensure that goods being imported and exported can still arrive at their intended destination. Secretary Pompeo said in a press conference that the border restriction would be reviewed after 30 days from approximately March 21, 2020.