Per the text, when Beowulf swims to the bottom of the "fiery lake" to kill Grendel's mom for killing Hrothgar's best friend, he destroys her by using the sword made from the Giants that was hanging on the wall:
"The wise ruler of the skies/decided justice easily when/Beowulf stood up again: there among the weapons/he saw a victory-blessed sword, an old sword made by giants/with strong edges, the glory of warriors. It was/the choicest of weapons,/good and majestical, /the work of giants, but/larger than any other man could carry to battle sport."
All that we really hear from Hrothgar is Beowulf brings back the hilt from the fight with Grendel's mom (because the sword has melted):
"Nor could I accomplish anything/with Hrunting, that strong/weapon, but the ruler of men/granted me to see/a beautiful old mighty sword/hanging on the wall."
We assume that when Hrothgar examines the sword hilt, he learns through the inscription and explains that it was an old heirloom created by a race of giants from before the Biblical flood. Per the text, we must conclude the flood destroyed them.