South Wales Police said around 50 officers are deployed in the "extensive" search for Ben Thompson, including a helicopter and search dogs.
Chief Inspector Eddie Ough said he believed Ben had left the city centre to try to walk back to where he was staying that night.
He asked people in the west of Cardiff to look in their outbuildings, as "Ben may have tripped, fallen, hurt himself, be unconscious."
More top news
Ospreys lock Lloyd Peers has joked that a change of name after marrying his wife, could mean better fortunes on the pitch.
Much of Wales is bathed in warm sunshine at the moment, but be warned! It is about to come to an abrupt end overnight!
A farming family in North Gower say sales of their Salt Marsh Lamb have rocketed, thanks to TV chefs who rate the meat's distinct taste.