The parking brake engages every time you push the clutch pedal down, and if the pins behind the small brake lever that the linkage hooks to get rusted the brake won't disengage. Sounds like it may be time to let a dealer look at it, since you indicated that you don't know were the drive belt is.
Pull the hydro disengage lever on the back of the mower, and then see if you can push the tractor. if it won't push then the parking brake is stuck.