fall on (one's) knees
To kneel down, usually in a show of respect, fealty, awe, subjection, or reverence. The members of the congregation all fell on their knees to begin their morning prayer. I fell on my knees at the sight of the devastation.
fall on one's knees
and fall to one's kneesto kneel down, usually in respect. The people fell on their knees and prayed in gratitude for their salvation from the flood. They fell to their knees in awe.