How can i fix my toyota estima 3L?

My mechanic broke the radiator lid and i discovered it after a couple of weeks and found that air has got into the radiator. I replaced a new lid and its running now but the engine seems to rattled and shaking if start. I checked the spark plugs and found out that they were all wet and replaced them. However, my mechanic said he thinks there are only 5 cylinder operating..how can I fix this problem...i drove to Dunedin in the weekend with my family and was very sad to find that the van has lost power as it climbs up the road..what is happening to the car engine??? how can I fix it??