A man who was attacked in Abington Park in Northampton was found in a driveway, unable to remember how he got there.
A spokesman for Northamptonshire Police said: "The incident happened between 5.15pm and 5.45pm on December 11, when a man was sat on a bench between the café and the bird cages when he was attacked from behind.
"He was later found on a driveway in Wellingborough Road, Northampton, but cannot recall how he got there."
Witnesses or anyone with information should contact Northamptonshire Police on 101. Alternatively, they can call Crimestoppers anonymously on 0800 555 111.