2001 Dodge Dakota Spark Plugs

I own a 2001 Dodge dakota 6 cylinder truck. A friend who tells me he is a ASE certified mechanic changed my oil, which included the oil filter and air filter. I also asked him if it was an easy task to change the spark plugs. He said it would be no problem. After he changed the spark plugs I was driving mt son to school and the truck barely made it up the hill. After that my truck would barely go over 10 mph. I got home and he looked at it and said they needed to be lined up. Well that helped the truck go a little faster 40mph and still cannot go up hills. What could be the problem? I'm taking my truck to a real mechanic that has always fixed my vehicles beautifully tomorrow, but I would like to know what he could have done wrong to cause my truck to act like this. Thank you for your help and advise.