What's the easiest and fastest way to replace a missing passport? I need to travel to Canada in two weeks.

Go to the US passport website: www.travel.state.gov for the information on replacing a passport. They offer expedited service if you are willing to pay extra.

If you are a US citizen you don't need your passport.

You will, however, need:

1.
Proof of your U.S. citizenship such as your U.S. passport or certified copy of your birth certificate issued by the city, county or state in the U.S. where you were born. If you are a naturalized U.S. citizen and do not have a passport, you should travel with your naturalization certificate. A driver’s license, voter’s registration card or Social Security card is NOT valid proof of citizenship.
2.
Photo identification, such as a current, valid driver’s license.

Make sure you report your passport missing and file for a new one.