Once upon a time, Lord Vishnu was tired of fighting with the demons for 10,000 years. Then he went to his adobe Vaikuntha and there he placed his bow on the ground and sat in Padmasana went in yogic sleep . Due to strong sleep, he kept his head on bowstring. At the same time there was a plan to perform a yajna by the gods, then all the deities went to Vaikuntha to pray to Lord Vishnu. They saw that Lord Vishnu was in yoga sleep and they decided not wake him up thinking that he was asleep. But even after a long time, Lord Vishnu did not wake from yogic sleep.Devraj Indra asked the gods about awakening Lord Vishnu. Then all the gods decided to awaken them thinking that the yagna would not be complete without Lord Vishnu.
At the same time Brahmadev created a insect to awaken Lord Vishnu and ordered him to cut the string of the bow. But that insect said, Lord Narayana is also adorable to the gods, why should I do the disgusting task of awakening him from sleep. Unless someone has selfish interest in it, he will not do such an act. Brahmadev, tell me what I will get by doing this disgusting act. Then Brahma ji, knowing what is in the mind of that insect, said that the things which fall outside while performing the yagna is yours from today. Hearing this, insect agreed to cut the bow string and cut the bow string. When the string of the bow was cut, a terrible sound came because if it and suddenly there was darkness all around. Gradually when this darkness disappeared, the Gods saw that Lord Vishnu was present there without head, his head had fallen somewhere with the crown and coil. This caused a great fear to the gods and they were overwhelmed with worry about what to do next.
Seeing the gods immersed in such concern, Brahma Ji said that no work in the world is done without reason. That is why we should worship Bhagwati Yoga Maya, and , ordered the same Vedas which had taken bodily form to praise Bhagwati Durga. Then the Vedas started praising Goddess Adishakti. Goddess Bhagwati Parashakti was pleased to hear the praise of the Vedas.
As soon as Devi was pleased, goddess started speaking. Gods, you do not need to be afraid. There are two reasons why the head of Lord Vishnu has been cut. Firstly, once Vishnu was laughing by looking at the face of his wife Lakshmi. Then Lakshmi felt that maybe shri hari does not like me anymore, that is why he is laughing at me. At that moment, that Lakshmi Devi, having peaceful nature, got very angry and said to Vishnu, you may become headless. Due to the same statement, Vishnu’s head is cut today. Another reason in this is that a monster named Hayagriva has done my penance and has received a boon from me that he should die from the hands of Lord Hayagreeva. Now that monster is stronger because of my boon and is harassing Brahmins and Sadhus in the world. It is now necessary to end his life. That is why, to make vishnu horse headed, this why his head is chopped off today.
Later, Brahma ji cut the head of a horse and placed it on the head of Vishnu according to the order of the goddess, and according to the boon of the goddess, he finished the monster by name as Hayagriva.