There are many kinds of defects that could make a rental unit unlivable. The implied warranty of habitability requires landlords to maintain their rental units in a condition fit for the "occupation of human beings."
Even if a rental unit is unlivable, a landlord may not be legally required to repair the condition if the tenant has not fulfilled the tenant's own responsibilities.
Generally disputes are best resolved between the renter and property owner. Assistance in mediating landlord-tenant disputes may be available.