Ryobi string trimmer head not turning.

The head on my string trimmer won't turn with any power. I can stop it with my finger. I removed the flex shaft and it appears to be unbroken. I removed the shaft parts covering the clutch end of the engine exposing a round metal part that the flex shaft fits into. I tried to hold it while running the motor at high rpm and could not do so which makes me think the clutch is okay. I also removed the head assembly, exposing the part the flex shaft fits into. Inserted a large screwdriver into the exposed hole the shaft fits into and tried to hold it while manually rotating the head and could not so it appears that part of the trimmer is okay. I've never worked on a string trimmer before and am at a loss as to why it's not working. It's a Ryobi 767 rj straight shaft trimmer. I'ts only a couple of years old and I would hate to have to replace it so soon. Hopefully someone can provide some insight. Thanks.