What is the possibility of coolant leaking into the fuel and how ?

Our 2001 Toyota Camry was running without any issues before this incident that
happened one evening when I was returning from work from another city 120 miles
from home. After driving for about 60 miles in the freeway suddenly saw steam
escaping from the front hood continuously. I continued to drive 10 miles slowly to
arrive at a city and finally had got it towed to a nearest automotive repair garage. They
changed radiator and thermostat in a day and I drove back another 60 miles home.
After I parked in the front porch the car could not be started again at all. When the key
is turned on to ignition is heard but does not get the car started. So had to tow the car
to the service station where they diagnosed that coolant has been leaking into the fuel
and suggesting to change the engine. It is hard to understand how this could have
happened?