Line 62 of Part II reads,
Twelve years did the Scyldings' sovereign bear this trouble, having many woes and unending travails
and indicates that Grendel, a monster who has made his home in a subterranean region near to the Heorot Hall, Danish King Hrothgar's mead hall, becomes disturbed by the presence of the warriors who eat and drink, and he takes extended actions against the warriors. Finally, in the night he goes to the hall, curious to discover what they do when they finish their drinking. When he sees them sprawled fast asleep and unsuspecting of any trouble, Grendel capitalizes on the situation and "snatches" up thirty men, crushing them, and carrying their bloodied bodies back to his lair. Waging war on the Danes, he returns the next night and again and again, killing more men; he returns until Herot Hall is empty: "So he reigned in terror and raged nefariously against one and all until that majestic building stood empty."
...Though he lived
In Heorot, when the night hid him, he never
Dared to touch king Hrothgar's glorious
Throne, protected by God--God,
Whose love Grendel could not know. (II, 81-85)
Hrothgar is feckless (unable to achieve anything) against this monster because Grendel will not take ransom or any bribes as other (normal) medieval warriors would. It is not until Beowulf hears of Hrothgar's plight and decides that he will go to help the Danish leader that changes occur at Heorot, having been empty those long twelve years.