engine problem

[robotype]

my car is stock (swiss zenki) and ran 250.000 km. in the last week she had some problems starting, like if one piston was sleeping but after that it worked fine. Finally, today I was in the highway and around 3000rpm I hear a sound tac tac tac tac tac coming from the top of the engine, sound got quickly louder and louder...and then I noticed the car won't accelerate...temperature gauge was ok though so I stop it, check the engine, it was actually quite hot as the liquid in the reservoir was boiling and spitting out for like 5 mins! maybe temp gauge is gone... anyway the only thing I could do on the side of the highway was checking the spark plugs and one of them was very dirty of oil so I swapped them and tried to start the engine...it started but one piston is definitely not working so I went back home very slowly. where should I start?