These steps go from harmless to drastic. Do them in order and stop when the app works, because the last ones cost you data or settings.
1. Close it properly and reopen
Not the back button. Open the recent apps view, swipe the app away, then open it again. An app left running for weeks can end up in a confused state that a real restart clears.
2. Restart the phone
Genuinely off and on again, not just locking the screen. This clears a surprising number of these.
3. Check for an update
Open the Play Store or App Store, search the app, and update it if there is one. Crashes that started suddenly for everybody are usually a bad version, and the fix is usually already published.
If the crash started right after an update, that is the opposite situation. On Android you can go to the app's Play Store page, tap the menu and Uninstall updates if it is a built-in app. For others you have to wait for the next version.
4. Clear the cache
Settings, Apps, find the app, Storage, then Clear cache. This deletes temporary working files only. You stay logged in and lose nothing.
On that same screen is Clear storage or Clear data. That one wipes the app completely: logins, settings, offline content. It often does fix the crash, but treat it as step 6, not step 4, and make sure you know your password for that app first.
iPhones have no cache button. Offload the app instead: Settings, General, iPhone Storage, tap the app, Offload App, then reinstall it from the App Store.
5. Check you have free space
A phone with almost no storage left makes apps crash in ways that look like bugs in the app. If you are under about a gigabyte free, fix that first with the storage guide, then come back.
6. Clear the app's data, then sign in again
Now do the step from the warning above. Make sure you can get back in: know the password, and if the app uses a code by SMS, have the SIM in the phone.
7. Uninstall and reinstall
This gives you a clean copy of the app and clean data. For WhatsApp, back up your chats first, otherwise you will lose them.
8. Check the phone is not too old for it
Apps drop support for old Android and iOS versions constantly. If the Play Store says the app is not compatible with your device, or it installs but crashes instantly on an old phone, that is the answer and no amount of clearing will change it. Check Settings, About phone for your Android version.
When it is every app, not one
If several unrelated apps crash, suspect the phone rather than the apps:
- Storage full. The most common cause by far.
- A system update that went badly. Check for a newer one.
- Overheating. A phone that is too hot throttles hard and kills apps. Let it cool out of the sun and try again.
- Safe mode test. Hold the power button, then press and hold Power off until the phone offers Safe mode. In safe mode only the built-in apps run. If everything behaves in safe mode, something you installed is causing it, and the culprit is usually the most recent thing you added.