How to handle an internet outage and prevent future outages - updated

What causes an internet outage or Intermittent issue?
All kinds of things can lead to an internet outage, and many of them are out of your control:

Hardware issues, like damaged or disconnected patch cables, faulty network hardware, improperly grounded equipment, or a damaged circuit (which can be on your network or your ISP’s network!)
Configuration issues, such as a down interface link on your router or an improperly configured firewall (again, this could be on the hardware you manage, or changes your ISP’s network engineers are making)
Line issues, such as downed wires due to grounds maintenance or weather
Network congestion on your ISP side, where too much-localized traffic on the network causes your connection quality to deteriorate or fail
Hackers penetrating the network
Vandalism, like the cut fiber optic cables that caused major outages in San Francisco
Network infrastructure attacks, where network appliances are overwhelmed by a planned, synchronized influx of incoming traffic (DDoS or dedicated denial of service attacks)
Natural disasters, like floods, hurricanes, tornados, and hordes of locusts (sometimes these can even cause regional issues that affect areas hundreds of miles from the disaster itself)