Napoleon reaches a stalemate in one his battles sometime before the invasion of Russia and as a result is forced to slow down a bit and ends up dying in Paris still an Emperor with a truncated Continental System ruling over Europe. The years of revolt and British incursion have dislocated many and prevented harvests from being collected and the cut off of trade with Britain and sometimes harmful rules of the Continental System excaberate the situation.
Several years into Napoleon's heir's reign as Emperor, famine begins from the cumulative effects of this combined with harsh winters. The Great Continental Famine is made worse by his heir's less experienced administration and French self interest at the expense of its European vassals.
Numerous deaths due to starvation, regional rebellions against Bonaparte hegemony and foot riots occur, as do large scale emigration to the USA and the newly founded South American republics and Mexico on a lesser scale.