Almost immediately after the Geats and Danes celebrate their freedom from Grendel in the great hall Heorot, we read that
And his [Grendel's] mother, still/greedy and gallows-minded wished to walk/a sorrowful journey, to avenge her son's death. (ll. 1276-1278)
As we know, she seizes one of Hrothgar's most important men, Aeschere, and carries him off. Beowulf, Hrothgar, the Geats and some of the Danes discover Aeschere's head as they search the fens for Grendel's mother.
When they finally discover the dark lake in which Grendel's mother lives, Beowulf puts on all the armor lent to him by Unferth, son of Ecglaf, who did not want to follow Beowulf into the lake--thus branding himself a coward. Beowulf, always the model of an Anglo-Saxon warrior-king, reminds Hrothgar of an agreement they made:
if I die in your service should lay down/my own life, that you'd always be/like a father to me when I've departed./ Be a protector/ to my young thanes, my close comrades. (ll. 1476-1481)
Having assured that his followers will be taken care of if he dies fighting Grendel's mother, Beowulf immediately dives into the lake to seek her.
After hours of waiting and watching, the group of Danes and Geats around the lake, seeing that "the waves were bloodstained," assume that Beowulf has been defeated and leave. Beowulf's thanes, however,
sat sick in mood/and stared at the mere;/they wished without believing,/that they would see their friendly lord. (ll. 1602-1605)
Given the time and evidence of slaughter, Beowulf's thanes assume, like the Danes, that Beowulf has been killed, but, as loyal retainers, they don't leave because they are still wishing but not believing that Beowulf is alive.
To reward their loyalty and patience, Beowulf surfaces with evidence of his victory and
they rejoiced in their lord,/and they thanked God/that they were able to see him sound. (ll. 1627-1628)
So, from a belief in utter defeat, Beowulf's men move to an intense celebration of his victory. But throughout the entire battle, Beowulf's men, young as they are, demonstrate loyalty and perseverance despite the fact that, for a time, they believe their leader is dead.
Beowulf's thanes, therefore, demonstrate exactly what Anglo-Saxon warrior society expects of a chief's followers--loyalty under even the most devastating circumstances and unconquerable belief in Beowulf's abilities.