Please explain the statement, " gene is hybridized with a radioactive probe and 'autoradiography' is used for detection. The regions where mutation is present in the gene will not appear in the photographic film since probe will not be able to bind with that part."

DNA Probe is segment of DNA and its sequence can vary depending on the sequence that has to be probed or to be enquired about. The gene of interest (in hybridization techniques of molecular biology) is hybridized with the radioactive probe (contain fluorescent markers) to find target complementary sequence in the genome. 
Autoradiography is used for detection. It is a technique of visualising the molecules that have been radioactively labelled. The photographic film shows the result of the binding of probe with specific regions on gene.
Probe does not bind to regions where mutations are present and so the regions on the gene with mutations can be detected.​