The story of Krishna's death:

This event is told in the Mausala Parva of the Mahabharata, but what led up to it starts earlier, in the Stri Parva. Right after the Mahabharata war, when Gandhari went to the battlefield of Kurukshetra where her sons' bodies were lying dead...

She was overcome with anger at Krishna for not stopping such a deadly war, so she gave him the following curse:

Krishna accepted the curse willingly, because it was all part of his divine plan to eliminate the Yadava race who had become a burden on the earth.

But the sequence of events immediately leading up to the Krishna's death is described in the Mausala Parva. 36 years after the Mahabharata war, The Vishwamitra, Kanva, and Narada came to Dwaraka, & Krishna's son Samba along with his friends decided to play a trick on the sages.

They dressed up Samba as a woman, and asked the sages what kind of son this woman would have. The sages saw through the disguise, and enraged by this act of disrespect, they issued the following curse:

And indeed, the next day Samba really did give birth to an iron bolt! When Ugrasena (the king of Dwaraka) heard about this, he ordered his men to make the bolt into a fine powder and then throw the powder into the sea.

And the production of alcohol was banned, because the curse mentioned that the Yadavas would destroy their own race while drunk.

But then shortly thereafter, the entire Yadava race went to the sea coast to bathe in the sacred waters of Prabhasa, and there they started to drink.

A drunken brawl soon ensued, and lacking weapons they started plucking Eraka grass out of the ground, which magically transformed into iron bolts with the power of thunder: You see, some of the powder from the iron bolt of Samba had washed up on the shore of Prabhas and...

...had magically grown into that Eraka grass.

In any case, using the weapons furnished by the grass, the Yadavas soon annihilated each other. Then Krishna's brother Balarama started engaging in meditation and soon gave up his body, turning back into Vishnu's serpent Adiseshan:

Krishna then sat down in the forest, knowing that it was time for him to depart the earth. He was shot by the hunter Jara, who mistook Krishna's left foot for the mouth of a deer:

Jara's shaft was made of the remaining powder of Samba's iron bolt,

Which had been swallowed by a fish, where it had solidified into a piece of iron. Jara caught the fish and made the iron into a shaft for hunting. Thus, just as the sages had prophesied, Krishna and his Yadava clan were destroyed by an iron bolt.

By the way, it is said that Jara was the reincarnation of Sugriva's brother Vali, whom Vishnu had killed from behind in his incarnation as Rama, so Jara was killing Krishna the way Rama had killed Vali.

The above answer was written by Keshav Srinivasan 6 years ago who is moderator of Hinduism stackexchange.

Many thanks to him :)