Why did my iPhone stop counting steps?

Check Health first - Search, then Activity, then Steps. If today is empty in Health, the usual cause is that Fitness Tracking has been switched off in Settings, under Privacy and Security, Motion and Fitness. If Health has numbers and only your app shows zero, the app has lost its Health permission.

How do I turn step counting back on?

Settings, then Privacy and Security, then Motion and Fitness, and switch on Fitness Tracking. That is the phone-wide master switch: with it off, no app on the device records steps.

Why does my step app show zero when Health has steps?

The app cannot read Health. Open Health, tap your picture in the top corner, then Apps, find the app and make sure Steps is enabled for reading. Reinstalling an app or restoring a phone from backup often leaves this permission behind.

Does Low Power Mode stop step counting?

No. The motion chip keeps counting and the data is all there. Low Power Mode holds back background work, so apps and widgets update later than usual - which can look like a stopped counter until you open the app and find the number intact.

your iPhone stopped counting steps

Before changing any setting, find out which thing stopped. Open Health → Search → Activity → Steps and look at today. If there are numbers there, your phone is counting perfectly and the problem is the app in front of it. If today is empty, the phone isn't counting, and nothing you do inside an app will fix that. The two branches share almost no answers.

Branch one: Health is empty too

Then the counting itself is off. In the order worth trying:

Motion & Fitness. Settings → Privacy & Security → Motion & Fitness. Fitness Tracking is the master switch for the whole phone; with it off, nothing records steps and every app shows zero. It gets turned off by accident, and by battery-saving advice on the internet.
The phone wasn't on you. Unglamorous and by far the most common. The chip records what it feels; a phone on the desk feels a desk. If yesterday looks normal and today looks empty, start here before anything technical.
Restart it. Genuinely — the motion path can get stuck in a way that a restart clears and nothing else does. Two minutes, no settings changed.
Check the storage. Settings → General → iPhone Storage. A phone with nothing free struggles to write new health samples, and step counts are the smallest, quietest casualties.
Check the date. A phone whose clock has jumped will file today's steps under a day that isn't today, and today will look empty while the data is fine.

Branch two: Health has numbers, the app doesn't

Then the counting is fine and the app either can't see it or isn't asking properly.

Health permission. Settings → Privacy & Security → Health → the app, or from the other end: Health → your picture in the top corner → Apps → the app. Steps must be on for reading. An app can be installed, opened and logged into with this quietly off — it was a single tap during setup, often months ago.
It was granted, then revoked. Restoring a phone from backup, or reinstalling the app, can leave the permission behind. The app looks familiar and starts from zero.
The app keeps its own count. Some don't read Health at all, or read it and then add their own samples on top. That shows up as a number that is wrong rather than missing — usually too high. Why your step counts don't match takes that mechanism apart.
Background App Refresh. Settings → General → Background App Refresh. With it off, an app can be perfectly correct the moment you open it and stale every other moment — which reads as "not working" from the home screen.

Branch three: the app is right, the widget isn't

A widget showing an older number than the app is not broken in the same sense. iOS spends a daily budget of widget refreshes on your behalf, and a widget can be a few minutes behind by design. Hours or days behind is a different matter, and the difference is worth knowing before you delete anything: why your widget shows an old number.

What Low Power Mode does and doesn't do

It is the usual suspect and it is usually innocent. Low Power Mode does not stop your phone counting steps — the chip keeps running, and the numbers are all there when you look. What it does is hold back background work, so apps and widgets update later than they otherwise would. If the count reappears intact the moment you open the app, you were looking at a delay, not a loss.

Straight to the likely cause

What you're seeing, and where to look first
what you're seeing	look here first
every app shows zero	Motion & Fitness is off, phone-wide.
one app shows zero, Health doesn't	That app's Health permission for Steps.
the app is higher than Health	It is adding raw samples instead of asking for the merged figure.
the widget lags the app	Normal within minutes. Beyond that, the app's refresh behaviour.
it stopped on one specific day	A restore, a reinstall, or a permission tapped away that day.
counts are low but not zero	Nothing is broken. The phone was in a bag on the chair.

The steps you have already lost

They are not recoverable, and any app offering to estimate them for you is inventing numbers. This is worth being firm about: a step count is only useful while it means something specific, and a total padded with guesses about a morning nobody measured means nothing at all. Fix the setting, carry the phone, and let yesterday be short.

Where we stand

Angel Step has one number and one source. It asks Apple Health for the merged daily figure and shows exactly that, on the screen and in the widget — so when something does go wrong there is one place to look, and the answer is either "Health has it" or "Health doesn't". It never invents a step it wasn't given.

It isn't on the App Store yet. If your counts are not missing but merely disagreeing across devices, that is a different problem with a proper explanation: why your step counts don't match.