A billboard shall be considered a discontinued sign if it has carried no message for a period of 180 consecutive days, or if such billboard no longer identifies a bona fide business, commodity, service, entertainment or facility, or if more than 50% of the message on such billboard has deteriorated to the point that it is not clearly discernible from the road or street to which it is oriented. A billboard that has been discontinued shall be presumed to be abandoned and shall constitute an illegal sign. Any period of time for which the discontinued use of a billboard is proved to be caused by government actions, labor strikes, material shortages or acts of God, and without any contributing fault of the owner of the sign or user of the sign, shall not be calculated toward the number of days of discontinued use. Any discontinued billboard shall be removed at the expense of the owner of the sign. In the event that the owner of the sign cannot be ascertained after the City's reasonable inquiry, the discontinued sign and structure shall be removed at the expense of the owner of the property on which the sign is erected.