mk3 prob

i have a mk3 that has no power left in it.when i start it it knocks a lil at first then it will stop and the car will idle.then when u take the car out of park and put it in drive and try and give it some gas it acts like there is no power u can hear the eng tryin 2 go but nothing.i know the lady i bot the car from just put a topend on it.so should i just tear the motor down and see whats wrong with it or what?????