The story
Colonel arrived in English in the 1540s from Middle French coronnel. The French had borrowed it from Italian colonnello, the officer who led the small column of soldiers at the head of a regiment — from colonna, "column", ultimately Latin columna, "pillar".
On the way into French, the first L became an R, a common kind of change when two L sounds sit close together in one word. So the English word was at first written and pronounced with an R: coronel.
In the late 1500s, French spelling was reformed back toward the Italian original, and in the 1580s English writers followed, spelling it colonel. For about seventy years English speakers used both an R and an L pronunciation. The R form won, becoming standard around 1650 — but the new spelling had won too. Over time the word shrank from three syllables to two, giving the modern KER-nul.
So colonel is not a case of a sound that was lost. It is a spelling from one language attached to a pronunciation from another.
