Weed Eater One

I own a weed eater one WE261 riding mower, that I purchased last summer. I was cutting grass last week. When I was turning to make the next path I noticed that the blade stopped turning. I checked the belt and the belt looked ok. I also looked underneath and nothing was blocking the blade from turning. So I took the old belt off and replaced the belt with a new belt. After replacing the new belt the belt keeper was touching the belt on the side with the other pulleys. The belt is on and it is loose, but when I tried engaging the blades it tightens up. So I tried running the lawn mower with the blade engaged, it still does not turn. So I am my wits end and don't know where to turn. Can you please HELP!!!