I have a Trek mountain bike that has been hanging in the garage for over a year. I tried to ride it this weekend and found the crank arms move freely in the forward direction, just the same as they do (and should) in the backward direction. In a stationary position, you can turn the pedals forward but the rear wheel is not engaged. Chains are on the gears, and it doesn't matter which gear I'm in. Can anyone tell me what is likely the issue? Thanks from a Newbie.