My GHD's are making my hair smell bad (they never used to)?

so my lil sister had a friend over last week and she and her friend used my straighteners without asking and after her friend straightened her hair it smelt like burning hair/grease or something ewww! and i tried to wipe the straighteners after she went but now whenever I use them my hair smells really bad and it never used to! wtf? my hair is clean, how do I clean the straighteners properly? or do i have to buy new ones?