Happened to me once. I flew from HKG to YYZ via NRT and due to weather the NRT -> YYZ flight got cancelled. The communication at the airport is going to be absolutely terrible but they will make arrangements for you to stay overnight somewhere in the city. When it happened to me we stayed at the Sheraton Tokyo Bay near Disney Land.

You will have to clear customs and with a US Passport I'm sure you won't have any issues getting a Visa on the spot.