I though you understood it overheated.

Since it happened suddenly, it either:

    had the water pump fail (check it)

    or

    Lost coolant

    or

    Started to freeze up



The thermostat would not cause a loss of inside heat.
If it was really cold when it happened maybe you didn't have enough atifreeze and the coolant turned to slush.

When you get it back together keep a close eye on it.

"Those who cannot remember the past are condemned to repeat it" -George Santayana