Why am I getting an error message when I try to flash my phone?

[AC Question]

need help immediately

hey please help. i wanted to flash factory images 5.1 lolipop in my nexus 5. but i used nexus 5 root toolkit to do that. so what the phone turned on it was a message "unfortunately phone has stopped"and i couldnt do anything. then i did a factory reset and the message "unfortunately phone has stopped" still appeared but it was another message saying Nfc service has stopped. i dont know what to do. please help me.