Why won't my phone connect to Wi-Fi?

In order or probability:

• Wi-Fi is turned off
• Signal is weak
• Wrong network is selected
• Wrong password/credentials
• Saved network profile has the same name but wrong configuration  (Try to delete or "Forget" the network and connect again)
• Signal interference
• No IP addresses available from DHCP server
• Damaged antenna
• Damaged radio
• Disconnected antenna wire
• Firmware bug
• Damaged Wi-Fi chipset
• Sunspots
• Aliens
• Electromagnetic pulse from a nuclear weapon
• A hex or curse placed on your phone