This is hard to say, given the breadth of the Middle Ages and the state of visual depictions of common people -- unlike with photography, there's no chance of accidentally snapping a candid image, each image must be composed by an artist and that artist brings their own conventions and prejudices to the table. But we can still get an idea of what non-noblemen and non-noblewomen were doing with their hair from artwork.
For example, we can look at an array of working men from the 15th century in the context of works like the Hausbücher der Nürnberger Zwölfbrüderstiftungen -- these are all craftsmen, which situates them as commoners but not as the poorest of the poor. They are also illustrated in a certain style, with certain stylistic conventions to suggest certain features and an over-representation of certain age demographics (lots of white-haired balding men) but together they're at least a collection of individuated working men, and these illustrations are meant to be likenesses of specific men.
Even in the case of balding men, these are clearly haircuts -- they're not hair allowed to grow wild, they're not long hair tamed with a braid, furthermore they are men's haircuts designed to differentiate their wearers from women as much as their beards do. Even within this fairly homogeneous group you can see a variety of styles among men who were not otherwise ultra-fashion-forward -- forked beards vs. non-forked beards, just how long or how cropped do you wear your shortish hair, what to do with your hair when it began to go bald, hair swept back vs. falling forward, and so on. "Just this specific collection of 15th century Nuremberg tradesmen" is an incredibly small sample size, but in general, across the High and Late Middle Ages men had shorter hair than women, generally unparted. They might wear beards or they might not. What was normal for individual commoners varied by their region and its trends, and there were variation within those bounds; in the most general possible terms, women wore their hair as long as was practical for them, tying it back or braiding it but generally keeping it tamed under some kind of hair-covering even when working. Hair might be cut with knives or with hinged scissors; men might trim their beards as well as their hair with razors that were at times basically just small knives.
Another selection of working men and women:
Trying to track down bare-headed depictions is a pain, because head coverings in general were ubiquitous throughout the Middle Ages in Europe -- this was both for the sake of practicality (if it's rainy and cold, you'll want a hood, if it's sunny and hot in the fields, you'll want a wide-brimmed hat) and for the sake of contemporary notions of decency; adult women covered their hair for reasons of modesty, and people with their hair totally uncovered in day-to-day contexts were considered disheveled, abjectly poor or incredibly hasty or even mad. People even covered their hair in the bath. Head-coverings grow more elaborate in accordance with one's position on the socioeconomic ladder, but working-class women had recourse to all manner of coifs, hats, hoods, and veils, governed by the fashions of their own places and times; these were secured with straight-pins as well as tied in place. Totally uncovered hair was never the norm for adult women. You'll sometimes see "uncovered hair on adult women was for prostitutes" in overviews of medieval hairstyling, but this is more of visual shorthand for their licentiousness than anything reflecting reality; plenty of medieval prostitutes across classes owned and wore hats, veils, and head coverings, across the spectrum from bare poverty to total splendor.
Also some past answers might be of use to you here:
