I see from the tags you have a LT1042. These are as we call them big box entry level Lawn Tractors. Being honest I have not worked on one of these. About all I can see for possibilities are a problem with the belt, the hydro release being in the released position or a problem internally in the trans/hydro unit. This is a link to the MTD manual where you can download the manuals for your tractor. You'll need the model and serial number.
http://manuals.mtdproducts.com/mtd/Public.do