What, only one? But there are so many possibilities! Here's two:
Etruscan. POD 509BC.Lars Porsenna of Clusium conquers Rome, reinstalls an Etruscan dynasty. Rome is 'Etruscanised' and becomes just another Etruscan city. One of the members of the Etruscan League eventually rises to prominence and unites the cities, perhaps in opposition to Gaulish incursions into Italy, into an Etruscan-speaking empire, which survives various vicissitudes to encompass most of the Italian peninsular down to the present day.
Cornish. AD1548 Glasney College is not destroyed but is secularised, and many old manuscripts are preserved there. There's more interest in the reformation in Cornwall in 16th century; during the reign of Elizabeth a translation of the Bible into Cornish is authorised - this is finally completed by 1629. The Prayer Book is also translated and church services are held in Cornish. A certain amount of secular literature is also written in the language. Cornish still declines, but much more slowly than in OTL. In the 20th century it is still the first language of many people in rural Penwith, giving a solid basis for the modern revival movement throughout Cornwall. In the present day it is used in education and the media; there are now five Cornish language primary schools, and in 2016 the first all-Cornish secondary school (Scol Michal Joseph Angof) was opened at Penzans.