Because it's quite common for fantasy games/stories to be set in a roughly medieval time period, it's best to do some research so your world fits the facts.
The Middle Ages and the Dark Ages are synonymous. The dates for this period are roughly 400-1500, basically from the the fall of Rome to the Renaissance. And that is broken down further into three periods: Early (500-1000), High (1000-1300), and Late (1300-1500).
The "Dark Ages" is a misnomer in that during this time there was actually quite a bit of information exchanged, particularly via trade with foreign countries.
Education wasn't limited only to cloistered monks. The later Middle Ages (1000-1500) were a time of philosophy, mathematics, astronomy, anatomy, engineering, etc.
Not all knowledge of the Ancient World was lost "in a single day and night of misfortune" like Atlantis. Much learning remained from Rome, Greece, and Arabia. And even more knowledge was introduced through contact with eastern Asia. Silk, spices, gunpowder, and decimals (just to name a few) came to Europe during this time.
Healthcare and sanitation were not as good as they are today (speaking
in regards to the sterile and shiny Western world), but it wasn't all
filth and squalor either. People took baths, brushed their teeth, and
washed their hands after doing their business. The cleanliness of the
water being used for these activities is what's really up for debate.