Is trying to hack into someone's instagram account for example or email a major sin?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The basic principle is that penetrating websites and hacking accounts is a form of aggression that is forbidden by Sharee’ah, so it is impermissible to do it without any legitimate justification.

Hacking accounts without justification involves aggression, spying, harming others, and harming a Muslim without any right. Some scholars have stipulated that each of these acts are from the greatest sins.

In addition, whoever tries to hack accounts as he can and is determined to do so, he is ruled as a hacker, even if he is unable to achieve the hacking.

Ibn Taymiyyah  said: “and also who is keen to sins and insists to do so and it is only the disability that prevents him, he will be punished for that with the same punishment of the actor” according to an authentic Hadeeth: “If two Muslims confront each other with swords, each of them wanting to kill the other, they will both be in Hell.” It was said to him: “O Messenger of Allah, (we understand about) the killer, but what about the one who is killed?” He said: “He was determined to kill his companion.” In another wording:” “He wanted to kill his companion.” What is meant by “wanting” is “determination”, and it is the firm will, that with it he found what was possible, which is: fighting, but he was unable to kill, and this is not a concern not to be written.” [End of Quote]

Allah knows best.