fall or drop to a lower place or level; "He sank to his knees."
5
collapse
break down, literally or metaphorically; "The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice"
3
weaken
become weaker; "The prisoner's resistance weakened after seven days"
2
faint
pass out from weakness, physical or emotional distress due to a loss of blood supply to the brain
1
sink
descend into some soft substance; "He sank into bed"
1
sink
go_under, "The raft sank and its occupants drowned"