Mental illness can have many different facets and causes. Take an otherwise "normal" functioning brain and introduce toxins or considerable stress, and the brain will defend itself in various ways that can alter your perceptions. Remove these external pressures, and eventually over time the brain can heal itself, even rebuilding pathways.
However, if the cause is genetic, or a defect at birth, the condition is irreversible and requires life long management.
I am not speaking of traumatic brain injuries, from which there is no return.