copied from an earlier answer of mine:
You speak of a peasant being able to climb the social ladder of the Middle Ages and rise the rungs to the highest position in the land, that of King. I'll do you one better though. How about a peasant (though we need to remember that peasant =/= poor) rising to become Basileus Rhomaioi, Emperor of the Romans, and the most powerful ruler in the world at the time of his apex of power?
Basil the Macedonian came from humble, though not necessarily impoverished, peasant origins in the Theme of Macedon in Thrace (the area between Istanbul and Northern Greece today) from uncertain ethnic origins. Anthony Kaldellis has made a lot of noise recently about the lack of ethnic variation in the Empire, and the primacy of the idea of Roman identity (meaning Greek speaking Christians who followed the Patriarch of Constantinople) overriding all others. Despite this, his origins are sometimes given as Armenian or Slavic and its uncertain what his exact background was.
His early life is somewhat unclear but he entered into the service of a prominent noble and made a name for himself through military action as well as adept social maneuvering and entering into the good grace of both the Imperial family and other prominent landowners. He was able to gain the favor of vastly more wealthy imperial figures who would patronize, and help fund, his rise to prominence. This culminated in his appointment as a member of the personal retinue of the emperor. However he was not done yet! He was even able to rise to the level of marrying the emperor's, Michael III, favorite mistress (though the son she had ostensibly with Basil, the future emperor Leo IV is, and was, widely believed to be the son of Michael III). Even this was not enough for his ambition and he turned on his own patrons rather deviously.
He had the emperor's uncle assassinated based on the fear of his own rebellion for the crown and was made co-emperor after the deed was done (perhaps to ensure that Michael's biological son would ascend to the throne), not satisfied with even this highest possible honor, he had Michael III murdered scarecely a year alter and became the sole ruler of the Byzantine Empire. Not bad for a peasant from rural Thrace eh?
However, as grand and rapid as Basil's ascent was we need to remember that his rise was enabled by people far more powerful than he was originally. His rise through the ranks was predicated on noble largess that he was able to tap into through his own considerable skill at arms and adept social maneuvering, but he did not grow from rags to riches purely on the basis of his own merit. However his story does show that social system of the Byzantine empire was not nearly as rigid as commonly imagined and that it was possible, if extremely rare for peasants to rise to the most powerful positions in the Medieval world.
In Spain we have one such example in the Late Middle Ages: Hernando de Zafra. In this case, Zafra is a geophraphic indication rather than a surname.
Hernando de Zafra was born in Zafra, in Extremadura, "of plebeian parents" according to Gonzalo Fernández de Oviedo, who had the occasion to befriend the man. His parents were peasants, but probably relatively well-off. Zafra studied Law at the University of Salamanca, as being a jurist was a good way of climbing the social ladder. He studied thanks to a scholarship, and soon his intelligence caught the attention of some of the higher-ups, which landed him a job. His continued good service and ability to understand the Laws and torture them got him in the good graces of the Monarchs, who appointed Zafra as one of the royal secretaries.
With this office, Zafra served as secretary, which meant acting as occasional diplomat as well, and made a fortune. He actively participated in the negotiations for the surrender of Granada, and latter negotiated with Abrahen el Cayçi and Yusuf ibn Kumasa the sale of Boabdil's lordships. This negotiations, along with some others in the grayest areas of the Law made him exceedingly wealthy, and earned him a lordship, and also being married to a woman of the house of Ayala, one of the oldest and most prestigious ones in Castile.
His palace in Granada still stands today.