Could you tell me what happens for instance, if a cancer returns?

?can be retreated?: Cancer can return as either a local recurrence in the same area as the original cancer or as a metastasis which is spread to other sites via the blood stream or through the lymphatics. Recurrences or metastases can sometimes be treated by surgical resection with possible cure or chemotherapy and/or radiation might be used.

Depends: Some cancers grow very slowly or spread only locally if they recur. More aggressive cancers can lead to metastatic disease or locally invasive disease.