In this year the Moon was eclipsed on mid-winter's Mass-night, and the same year King Ecgbert subdued the kingdom of the Mercians and all that was South of the Humber.Note the lack of panic, such as we expect from Hollywood's portrayal of technologically primitive people experiencing an eclipse. Even if your theory of the heavens were no more sophisticated than perceiving heavenly bodies as balls of light affixed to concentric crystal spheres, you would realize that they could simply overlap at times. The Babylonians and Greeks had figured out the patterns of eclipses centuries earlier than the Anglo-Saxon Chronicle, and Isidore of Seville (c.560-636) in his Etymologies (which was used throughout the Middle Ages the way we would use an encyclopedia) explained
"The moon suffers an eclipse if the shadow of the earth comes between it and the sun" while an eclipse of the sun takes place "when the new moon is in line with the sun and obstructs and obscures it."While Medieval Europe had Isidore to explain what was happening, however, they did not necessarily have the knowledge of the Babylonians and Greeks to understand why it was happening. The event could still be unnerving. Bishop Eligius of Noyon in the 7th century warned: "When the moon is darkened, no one should dare to utter shouts, because it becomes dark at specific times at God's command." Hrabanus Maurus (c.780-c.856), another encylopedist, tells of a lunar eclipse when some threw spears toward the moon, trying to defend it from its attacker.
Even if the mechanism of eclipses was understood, people might still accept them as a sign of great portent, or as the result of human actions. Bishop Thietmar of Merseburg, referring to an eclipse of 990, wrote "I urge all Christians that they should truly believe that this does not happen on account of some incantations by wicked women, nor by eating, and it cannot be helped by any action in the world."
Oh, and when Astronomy Today tells you that the eclipse of May 5, 840 so frightened King Louis that he "died just afterwards"? Ask to see their sources. Louis died on June 20th at the age of 62, after years of quelling civil wars. I think there are likelier reasons or his death than being afraid of an eclipse.