2001 Ford Crown Victoria Police Interceptor oil leak. What could be wrong?

We went to change the oil and there's oil in the coolant reservoir and there's oil covering all of the pipes and other components under the motor. This is the first time this has ever happened. What possible reasons are there for why this has happened, and what is the worst case scenario cost to fix this?

Maybe because some parts of the car were malfunctioning.

Could this be from a blown head gasket?

Locate the oil sending unit