Were eyeglasses already invented during the medieval period?

Apparently the Arabs invented some kind of corrected lenses for eyeglasses sometime during the European Dark Ages. Though the Europeans did not have eyeglasses during the Middle Ages. An Arab text entitled "The Book of Optics" published by Alhazen in 1021 was translated into Latin around 12th century. By this time Europe was coming out of the Medieval period. This Arab book was essential to the development of eyeglasses in Italy around the 13th century.

So in answer to your question, no there were no eyeglasses in Europe during the Medieval period.

There are records of reading glasses in the later part of the 1200's. What dates are you considering as medieval? You could use the Albigensian Crusade against the Cathars, but that takes place before the use of eyeglasses. Remember the line "Kill them all, let God sort them out"? That comes from the massacre of the Cathars. It is an interesting little known bit of feudal/religious history. Check the links.

You may be searching around to find a rare example of an Arab invention but glasses were Italian going back to medieval times