First you have to diagnose the problem. It could be one or more issues depending on the age of your mower. Use your mower manufacturer's website with exploded views of the parts of your drive system to become familiar with them. Is it front wheel driven or rear wheel driven? If the power drive is belt driven, the belt and or pulley could be worn and need to be replaced. It could be the transmission assembly or it could also just be the traction cable needs to be adjusted or replaced. These are the main components of most drive systems. Check out these youtubes. If you need to replace something, search youtube for that particular repair.
too generic of a question........what kind is it?
That's a tough question without seeing your mower. Try sending a photo. Maybe you'll be lucky someone might have the same mower with the same problem.