overheated PA

I'm affraid that I overheated the engine of my '93 PA. I haven't realised that it's loosing coolant (the 'coolant low' lamp works but it didn't turn on). After about 5 km the red warning lamp next to the temperature gauge turned on (the temp was about 90 - 100 C). I turned off the engine and I've found the tube that goes to thermostat is disconnected (I've no idea how to explain it in english). I've refilled the cooling system with about 6 liters of coolant. The engine is powerful, runs quiet and perfect as it did before - what do you think?Is it very damaged?

Not likely there's damage unless you ran the engine for an extended period without water. Your engine has a cast iron head which makes it resilient to overheating. If it were damaged, it would most likely be a blown head gasket.