Could un treated brain trauma as a result of hard beat on my head can cause a mental illness like shezsofrenia or depression ?

Yes -TBI : Traumatic brain injury (TBI) is associated with increased risk of mood disorders such as depression and anxiety, as well as psychosis and schozophrenia, especially among those individuals with a genetic predisposition toward these conditions. If symptoms have developed in someone you know, would recommend the affected individual be evaluated.

Brain traumas: require either a neurological exam or imaging, or both to be sure of the extent of damage, if any. Severe brain trauma causes lots of problems, most often some form of dementia (loss of cognitive functions--memory, calculation ability, etc.) It normally doesn't cause schizophrenia, but depression is common, especially if there is loss of mental function. Go to a neurologist, to start.