Wifi stopped connecting (Wemos D1 Mini)
Fixed.
My router is ancient and does not support WPA.  As of ESP8266WiFi library Version 2.5.0, WPA is is the preferred mode.  The solution is to get a better router, or add WiFi.enableInsecureWEP(); before calling WiFi.begin().

WiFi.enableInsecureWEP();
WiFi.begin(wifi_ssid, wifi_password);