Since @HankoTanks has covered the food, I'm going to take a look at lost technologies.
A lot of Roman era technology has already been lost by the 15th C. Plumbing, Concrete and Large scale architecture being the most obvious. You're now going to start losing a few others before they really develop, the biggest of these being key to the period.
Mining, Quarrying, Masonry, Blacksmithing.
These are going to become very regional skills at best. Of course Mining is always regional, there's no point having the skills away from the mines, but quarrying for stone, dressing that cutting that stone for constructing large buildings is going to be lost. Why? Population pressures and a lack thereof.
The primary use of stone was defensive fortifications and religious structures. Without human conflict there's no need for stone fortifications, wooden ones will keep out animals. Without high population, you can't afford grand religious structures. These key skills would slowly reduce in spread to the point where they're effectively lost to the population. Not entirely lost mind you, given the nature of nasty beasts in the woods, spearpoints and arrowheads would among the most valuable of trade goods.
and one that's liable to be more widespread than it was.
The English Longbow
You're right on the button for something like this to become a more widespread technology as opposed to the crossbow which superseded it by virtue of not needing a highly trained archer. In a smaller group, hunting is liable to become a more widely required skill, both for adding to your food supplies and dealing with nasty things in the forest.
Hopefully they won't lose agriculture, but I'd expect the low populations to slowly revert to stone age hunter/gatherer lifestyles, especially in wooded rather than rocky areas. Localised advances in hunting and fishing techniques offset by the loss of "advanced" technologies that require separate stages of production, mining/smithing, quarrying/masonry.
History of the Longbow
The English Archery Law of the 13th century ensured that English men would be come experts with the bow and arrow. In 1252 the ‘Assize of Arms’ ensured that all Englishmen were ordered, by law, that every man between the age of 15 to 60 years old should equip themselves with a bow and arrows. The Plantagenet King Edward III took this further and decreed the Archery Law in 1363 which commanded the obligatory practice of archery on Sundays and holidays! The Archery Law “forbade, on pain of death, all sport that took up time better spent on war training especially archery practise”. King Henry I later proclaimed that an archer would be absolved of murder, if he killed a man during archery practise! The victories over the French at Crecy, Agincourt and Poitiers were directly due to the expertise of English archers and the longbow. Skill in the use of the longbow took considerable time. The English invested in the time required – the French did not. Up to this point the skills and weapons used by a Knight were deemed to be worth 10 ordinary soldiers – hence the French reaction to defeats by the common peasant.