This recipe applies to Avalon 3.2 and higher.
In order to reprocess an already ingested file, you need to have the original master file at the place specified by it's file_location which can be different than it's absolute location. See Master and Derivative File Locations and Master File Management for more details on this.
mediaobject = MediaObject.find('avalon:1')
mediaobject.parts #Will return an array of master files associated with this item
masterfile = mediaobjects.parts[0]
masterfile.file_location #Verify that the file exists and is accessible at this location
masterfile.process #This will kick off the reprocessing