The man had been socialising in the Telegraph, West Street, Moulton, when he left at about 10.30pm on Friday.
He arrived home about 25 minutes later with a pain in the back of his head and was taken to hospital by ambulance and later released with minor injuries.
A spokesman for Northamptonshire Police said: “The man cannot recall how he sustained his injury and it is suspected that he was assaulted on his journey home, possibly in an alleyway that runs from Pound Lane towards Northampton Lane.
“Officers would like to speak to anyone who may have witnessed an assault late on Friday, 6 May, in this area, or who may have any information about this incident.”
Witnesses or anyone with information about this incident is asked to contact police on 101 or Crimestoppers anonymously on 0800 111 555.