I drove to the garage with my jeep and switched the engine off, when I tried to start it again it would start but not idle then would not start at all. I then realized the engine is locking.
I drained the oil and the strange thing is anti-freeze came out first then the oil. The oil and anti-freeze did not mix at all in the sump.
I suspect when it went for service they put the coolant in the wrong place. Could that cause the engine to seize?
Yes, it most certainly can.
The coolant is heavier than the oil, so the oil floats on top of the coolant (evidence when draining). The oil pump pickup is at the bottom of the oil pan. So, the oil pump was picking up coolant instead of oil and circulating it through the engine where oil should have been.
This starved the engine of proper lubrication at all bearing surfaces causing severe friction, galling, and seizure. The engine will need to be disassembled to see what damage has been done. Then it will need to be rebuilt or replaced.
Why the coolant was in the oil pan is a question we can not answer here. That could be a number of reasons like a cracked block, bad head gaskets, or put there by someone.
