This is a fairly broad question and the short answer would be that technology is always improving to some extent. The Middle Ages, which stretched from the end of Rome to the beginning of the Renaissance (5th-15th centuries), is usually considered a slow expansion of tech development - though I think this is a bit unfair as many techs advanced quite a bit; including iron work making the armor and weapons stronger throughout the era. The main reason for this misconception seems to fall under the guise of architecture as the Romans definitely had more pizzazz in their construction techniques.

I'll answer the question better with a concrete anecdote:
Concrete in the Classical era was strengthened during the Roman period by the use of volcanic ash which was readily available throughout their Mediterranean empire. This allowed the construction of massive public buildings that beautified the cities. During the Middle Ages the centers of trade moved north where this ingredient was less available, meaning concrete as a whole was less reliable, and why they turned mostly to stone and mortar, or wood for construction. The Renaissance brought with it the Age of Discovery which returned Europeans to the use of Roman techniques in architecture returning the beauty of antiquity.