After Beowulf severs Grendel's arm during the fight at Heorot, Grendel, knowing that his wound is mortal, escapes to his home in the fen:
Grendel had then/to flee, with his death wound, under the fen-cliffs,/to seek his sad home. For certain he knew/that his life's end had come, his day's number. (ll.819-823)
Even though Beowulf's defeat of Grendel becomes the occasion of great celebration among the Geats and Danes, who believe the threat to Hrothgar's people has ended with Grendel's death, a more serious threat looms in the form of Grendel's mother.
After Heorot is restored to its original grandeur (before Grendel's attacks began), Hrothgar's and Beowulf's men celebrate Beowulf's triumph over Grendel and sleep in the hall, completely unaware that trouble, in the form of Grendel's mother, is on its way:
She then came to Heorot where the Ring-Danes/slept around the hall. Then there was a setback for the earls, after in came Grendel's mother. (ll.1279-1282)
The men are so surprised by this attack that they don't have time to put on their helmets and armor--Beowulf is sleeping elsewhere--and Grendel's mother then
. . . quickly she seized one prince/in a firm grasp, then she made for the fen. (ll.1294-1295)
The man she takes, and later butchers, happens to be "the dearest warrior" to Hrothgar, who is inconsolable when he learns of his retainer's death, and he the asks Beowulf to pursue Grendel's mother, promising even greater riches if Beowulf kills her.
Beowulf's defeat of Grendel is actually just a prelude to the battle with Grendel's mother, who is animated by not only her hatred of men but also her desire for revenge and is much more powerful and dangerous than Grendel.