This happened to me on holiday this summer. Turned out to be a recognised fault with certain Fiats - the 5th gear is 'on the side' of the gearbox using oil scooped from the main gear box and it and/or the syncro wears. Parts - 5th gear and syncro cost me £470 from Fiat, fitted myself (average mechanic, no special tools, just confidence) in a day. If I do one again it will be a couple of hours. There was a link on a nother site from a guy who did this with photos (great help if you want to do it).
Fistral Beach (Fiat Ducato Swift Royale 1998), December 2010
