Ryobi unit will start but wont turn the chainsaw blade?

I have a Ryobi RY30040A weed wacker, trimmer, edger unit. We were cutting trees the other day and the chain saw was having trouble spinning. After changing the blade and all we were able to get more work done but now it wont spin at all. I can move the blade freely when its powered off but once I start up the unit it wont turn. I'm wondering did I strip the rod out of the center pole thing? I can buy an upper boom assembly but will that fix the problem?

Janet
September 2011