The 17-year-old was walking along the B5378 Shenley Lane, between St Albans and London Colney, when she was approached by a group of four people.
She was then struck to the head which left her unconscious. When she came round, she had no recollection of what had happened to her but was left with pain to her head and bruising to her arms. Nothing was stolen.
Detective Constable Steve Cook said: “I understand this incident may cause some concern in the community but I would like to reassure people we are doing all we can to establish what happened.
“The incident occurred near a busy stretch of road in broad daylight so it is likely there would have been a number of motorists around at the time.
“I am appealing for anyone who saw what happened or saw anyone acting suspiciously in the area around the times stated to please get in touch.”
Anyone with information is asked to contact police on the non-emergency number 101, quoting crime reference F3/16/909.
Alternatively call Crimestoppers, anonymously, on 0800 555 111 or through their Anonymous Online Form at www.crimestoppers-uk.org No personal details are taken, information cannot be traced or recorded and you will never need to go to court.