Drove car with reservoir cap off 25mi ran bad didn't register hot What damage? on 2000 Volkswagen Golf

had no heat changed thermostat-removed panel to check heater flappers they are okay. Still very little heat. Drove car forgot to put reservoir cap on and drove the car just barely made it home it ran so bad but gauge never read over 190.

Asked by for the 2000 Volkswagen Golf
seek daig on this one,and a headgasket chemical test