Mower wont move in any gear - Garden

If this is a ride on mower check the small rod at the back of the mower below (not under) the seat to see if it's out or in. If it's out then the whole unit has been disabled so you can push it across the floor. If it's in then you shouldn't be able to push it across the floor. If however, this is working like it should then you either have a belt issue if there is one or a tranny issue. Could be something as simple as an adjustment.