The term levee en masse was coined to refer to the citizen armies of the French Revolution and Napoleonic periods. This mass mobilization of the citizens was not often used in Medieval times.
Some would say that in early Medieval times there was a sort of equivalent of the levee en masse when the "barbarian" tribes (such as the Franks) invaded the Roman Empire, and their armies were thought to be made up of a general participation by most of the men of fighting age.
Whether such armies were actually comparable to a levee en masse is unclear. Anyway by the time of Charlemagne, warfare was increasingly the domain of an elite who could afford chain mail and horses.
Where a levee en masse might have been an appropriate term in Medieval warfare would have been in defense, particularly in defense of fortifications such as a walled city or town or a castle.
The common infantry (other than specialists such as longbowmen or crossbowmen) were not usually (there are exceptions such as Robert the Bruce's victory at Bannockburn) very important in open field Medieval warfare. Commoners and all could participate, however, in defending their village or castle or city against an attack.
In the South of France, by the 12th century, most peasants had moved from living close to their crops and fields in the fertile valleys to defensible villages on the surrounding ridges, and all these villages had built a castle (which housed a seigneur - or knight) to defend the village against raiders.
These must have been dangerous times. It was a lot of work to walk a long way down to the fields every day to work and back up every night. Peasants would not have done this if there had not been a real threat.
When raiders appeared, the women and children of the village would seek shelter in the castle, while the seigneur would arm the villagers (the weapons, probably spears, possibly shields and helms were probably kept in the castle, and formed the origin of the "stands of arms" which can still be seen as semi-decorative features hanging on the walls in some castles today).
The knight, possibly some members of his family (sons or brothers) and possibly some of his servants may have had some military training. The villagers would have had little, though some may have had hunting bows.
The knight would probably lead the villagers in defending the village. If pressed too hard, they would fall back to the castle. The castle could probably see and signal to several other castles in the surrounding countryside, and the defenders would hope that relief forces would harry the raiders supply lines, or attack them before the castle could be taken or starved out.
Cities under siege would also raise a levee en masse. This was done (unsuccessfully) in Crusader Jerusalem for example when it was assaulted by the forces of Saladin.
Indeed, the Crusader Kingdom of Jerusalem was one of the few Medieval realms to have institutionalized a sort of early levee en masse system. This was the right of the Kings of Jerusalem to issue the arriere ban, which obligated every free man to come to the defense of the Kingdom for a period of up to one year. (Only for service inside the borders of the Kingdom, not for offensive operations).
http://defendingcrusaderkingdoms.blogspot.com/2015/01/the-armies-of-outremer-in-12th-century.html