What happens if you leave Islam?

It depends on whether you mean spiritually or legally. Spiritually, Islam teaches that each person answers to Allah for their beliefs. Legally, classical law treated apostasy as an offense in a Muslim state, while many contemporary scholars argue that ruling was about treason or doesn't apply today. In the USA, UK and Europe, leaving any religion is a legal right, and no one may harm you for it.

Doubts are not leaving Islam

Having doubts, asking questions or feeling distant from your faith is not apostasy. Some Companions told the Prophet ﷺ about thoughts too grave to say out loud, and he called their discomfort with them a sign of faith (Sahih Muslim 132). Leaving Islam means consciously deciding you no longer believe, not struggling with belief. See What if I have doubts about Islam after becoming Muslim?

The spiritual view

Islam teaches that belief is a serious choice with consequences in the next life, and that judgment belongs to Allah alone. The Quran puts the choice in each person's hands: "whoever wills - let him believe; and whoever wills - let him disbelieve" (Quran 18:29). The door back stays open for life.

The classical legal position, honestly

The four Sunni schools held that a Muslim who openly left Islam could face the death penalty under a Muslim state. They based this on hadith such as "Whoever changed his Islamic religion, then kill him" (Sahih al-Bukhari 6922), and the hadith listing "the one who reverts from Islam (apostate) and leaves the Muslims" among three cases (Sahih al-Bukhari 6878). Even classically, it required a court, clear proof and a period to reconsider, and the Hanafi school did not apply it to women.

Many contemporary scholars read these texts differently:

Treason, not belief: the wording "and leaves the Muslims" points to deserting the community to fight it, in an era when changing religion often meant switching sides in war.
The Prophet's own practice: when a Bedouin repeatedly asked to cancel his pledge and then left Medina, the Prophet ﷺ didn't punish him (Sahih al-Bukhari 7211).
No compulsion: they argue Quran 2:256 means belief can't be forced in either direction.

Others still hold the classical view in theory. But every school agreed that no individual, family or group may punish anyone.

In the West

Freedom of religion includes the freedom to leave a religion, and it's protected by law in the USA, UK and Europe. Threats, violence or forced marriage over someone's beliefs are crimes. If anyone threatens you, contact the police. See Do I have to follow the laws of my country as a Muslim?

If you're a convert who's doubting

Slow down: don't make a big decision during burnout, loneliness or a bad experience with other Muslims.
Separate Muslims from Islam: sometimes the problem is a community, not the faith.
Ask your real questions: a patient imam or teacher should welcome them.
Keep talking to Allah: an honest dua for guidance is never wasted.

Key points

Doubting is not leaving Islam.
Classical law punished apostasy; many contemporary scholars link it to treason only.
In the West, leaving a religion is a legal right, and no one may harm you.
The door back to Islam is always open.

Tip: If you're struggling, tell one trusted person before you decide anything. Many converts find their doubts were really about loneliness or pressure, and those can be fixed.