If your Lawn Boy self-propelled mower won't drive when you pull up on the drive handlebar, you most likely have a broken drive belt that must be replaced. The drive belt wraps around the pulley above the blade under the Lawn Boy self-propelled mower deck and then around the transmission pulley on the rear axle of the mower. The repair isn't especially difficult.
Pull the sparkplug cap off of the sparkplug on the front of the engine to prevent the mower from accidentally starting while you work on it.
Tip the mower on its side so the gas tank is on top.
Unscrew the transmission pulley cover screw with the screwdriver. Pull the cover off.
Put the proper-sized wrench on the nut in the center of the tension pulley pressing against the belt wrapped around the transmission pulley. Pull out on the spring-loaded tension pulley to remove tension in the drive belt. Slip the belt off the transmission pulley.
Slip the belt off of the pulley above the blade. Slip the new belt onto the pulley above the blade.
Pull out on the spring-loaded tension pulley to allow the new drive belt to slide onto the transmission pulley with ease. Slip the new belt onto the transmission pulley. Release the tension pulley.
Screw the cover back over the transmission pulley. Tip the mower back flat on the ground. Put the sparkplug cap back on the sparkplug.