Good question. I am assuming you had an x-ray because of pain or some other symptom to make the diagnosis. Pars defect are usually either inherited, iatrogenic(caused during surgery) or degenerative. Sometimes seen in young adults who are extrmely active, i.E gymnastics. Typically if the disc is normal and the alignment of the two vertebrae are good follow up x-ray in 1 year should suffice.
Spondylolysis. This is frequently asymptoatic its prescence , a precursor for spondylolisthesis some can heal on there own, if this is from an acute recent injury then see a spine specilist for recommendation.