Lord Krishna, the 10th incarnation of Lord Vishnu, took birth on this earth with a purpose. His sole objective of taking human form was to cast away all forms of evil and cruelties from the Earth and bring in prosperity happiness and love.
According to some old beliefs it is said that Lord Krishna lived on Earth with his brother Balram for a span of 125 years. The first ten years of his life were spent in the towns of Gokul and Vrindavan, in Northern India. Then from his 11th year till he was 28, Lord Krishna lived in the land of Mathura. Soon he established Dwarka, a city in Gujarat and lived there for the rest of his life with his wife Rukmini and sons.
According to some old researches, Lord Krishna disappeared on 18th February 3102 BC. The period that starts after his disappearance is the period of Kaliyug. According some old traditional beliefs and notions, it is been said that Lord Krishna had once predicted that exactly seven days after his disappearance the Golden city of Dwarka in Gujarat will perish under the sea. This was what exactly happened as indeed Dwarka city is drowned in the sea due to some torrential rain. The whole incident was symbolic in nature for it paved ways to the beginning of demolitions and destruction. Thus was the beginning of Kaliyug. Though we not yet certain of the reasons of his disappearance but few reasons can duly assigned to Lord Krishna's leaving his spirit for the heaven. They are as follows:
The birth of Lord Krishna in the guise of ordinary being was on purpose, for His sole intention was to eradicate every kind of cruelty and evil power from the soil of earth. He wanted the Mother Earth to be free from brutality and vices. Soon this primary objective of His was superseded with a secondary motive which was to fight the historical battle of Kurukshetra so that true justice can be delivered to the Pandavas. Thus having accomplished his missions, it was time for the Lord to end his life journey on Earth.
Another significant reason for the disappearance of the Lord from his earthly abode might be the curse of Mother Gandhari. According to the stories from Mahabharata, the historical battle fought in Kurukshetra resulted in the death of all the hundred sons of King Dhritarashtra and Gandhari. So in order to offer his condolence to Mother Gandhari, Lord Krishna went to meet her but returned with volleys of curses. For he was cursed that just as the Kaurava dynasty ended fighting killing each other, similar outcome Yaduvanshi dynasty must face, for Mother Gandhari felt that Lord Krishna didn't play a fair role in putting an end to the war. Lord Krishna accepted the curse gracefully.
Another renowned folklore that seems to hold responsibility for the disappearance of the Lord is the story of Sage Vishwamitra and Sage Narada. For according to some famous epical tales it is said that once Sage Vishwamitra and Sage Narada visited the city of Dwarka to meet Lord Krishna. A few children of Lord Krishna dressed up Samba (the son of Sri Krishna and Jambavati) as a pregnant woman and asked the visiting sages to foretell as to what kind of a child Samba will bear. Feeling insulted, the sages cursed that Samba will give birth to an Iron rod which in turn will be instrumental in the fall of the Yaduvanshi Dynasty. The very next day, Samba gave birth to an iron rod but soon the rod was crushed to powder by Lord Krishna's sons and was thrown in the sea put in a bottle. However, one day the bottle was washed up the shore and the powdered iron took form of an arrow. Lord Krishna could foresee everything and as a last resort to save his Dynasty, prohibited liquor in the city of Dwarka. But to his disappointment the Yadavas became drunk and killed each other with iron rods. None of them survived. Thus the Dynasty perished along with Lord Krishna’s hope and aspirations.