Regain WIFI connectivity
My question is regarding disconnection due to failure/ power fail in
  wifi network - when such event happens, does wifi connectivity try to
  reconnect or to over come this possibility


use WiFi.setAutoReconnect(true); when initializing WiFi

other useful option is WiFi.setAutoConnect(). It controls if the esp8266 next time starts the WiFi connection with remembered setting, before your sketch starts.