It would be easy, and dishonest, to be vague about this. So let us be exact: no human has ever been revived from cryopreservation, and with today's technology, no one can be. This is the single most important fact to hold clearly in mind, because every honest argument for cryonics is built on top of it rather than around it. Cryonics is a bridge to a future that may be able to finish the job. Right now, the far bank is not built. Here is precisely why.
We cannot reverse the cause of death
Cryopreservation begins at legal death, which means the person arrives with whatever killed them unrepaired: advanced cancer, organ failure, the damage of a stroke. Reviving someone is not only a matter of warming them up; it requires curing the condition that ended their life in the first place. For most causes of death, today's medicine cannot do that even in a living patient, let alone a preserved one. Until the underlying disease is curable, revival cannot follow.
We cannot yet undo the damage from preservation
Preservation is not free of harm. The race against cellular decay means some ischemic damage accumulates before cooling, and vitrification itself is good but not perfect. Reversing that damage requires repair tools at the cellular and molecular scale that simply do not exist today.
We cannot rewarm without harm
This is one of the field's central unsolved problems. The cryoprotectants that make preservation possible are toxic at warm temperatures; they are only harmless while everything is paused in the cold. To revive someone, the body would have to be warmed cleanly through the danger zone without ice forming, and only then could those agents be removed without poisoning the tissue. No technology can do that for a whole human today.
We cannot yet read or restore the connectome
The bet of cryonics is that identity lives in the structure of the brain, the connectome described in memory, identity and the brain. Even granting that the structure is preserved, we currently have no way to map a whole human brain at the necessary resolution, nor to restore function from such a map. The candidate ways this might eventually be done are laid out in how we might achieve revival, but every one of them is a future capability, not a present one.
Why this honesty is the point, not a weakness
Stating all this plainly might look like undercutting our own case. It is the opposite. Cryonics is only defensible if it is sold for what it actually is. The claim is not "we will revive you." The claim is "we will preserve the structure that is you, as well as we can, so that a future medicine has something to work with, if it ever can." That is a real and honest offer. It also explains why the whole field rests on the lopsided logic of a small chance versus none: preservation cannot promise revival, but it can keep the door open that burial and cremation close for good.
Revival is currently not possible, and anyone who tells you otherwise is mistaken or selling something. Cryonics does not bet that we can revive you now. It bets that preserving you now is what makes a future revival conceivable at all.
How far off that future might be is a question nobody can answer responsibly, which is exactly why we treat timing with so much caution. What we will not do is blur the line between what we can do today, preserve, and what we cannot, revive.