my harvester will only go 0 miles a hour. any idea why
I had this same problem. The "cruise control" speed of vehicles is adjustable, you've inadvertently set the cruise control speed to 0. Not knowing what platform you play, I don't know what controls will adjust it up for you (on PS4 it is hold L1+R1 while cruise control is enabled, then move the left stick up and down to set the speed you desire).