No work on a musical instrument is ever "completed" in the sense that it does not have to be looked at ever again. 

A repair has a simple beginning and end: either the specific problem is fixed, or it isn't.  Examples: replacing a broken treadle strap; getting a silent reed to sound again; installing new grill cloth.

A restoration takes an instrument that is not playing, or is playing inadequately, and restores it from the treadles up, bringing everything into working condition.