The app will not start, or keeps quitting
Please do not delete the app. Deleting it removes the videos and images you saved, and reinstalling does not bring them back. Even while the app will not start, you can still get your files out — see below.
1. Check these first
Are you on the latest version?
You cannot check the version inside an app that will not start, so check in the App Store. Open the app's page there: if the button says Open you are on the latest version; if it says Update you are still on an older one. Crashes at launch are often already fixed in a newer build, so try updating first. The release notes show which version each fix shipped in.
Restart the device
When the device is low on memory, iOS can shut the app down before it finishes starting. Close other apps, restart the device and try again.
2. If it shows a white screen and then closes
When a very large amount of data has been saved, loading it at startup can take so long that iOS ends the app. One device that had grown to 330 GB of saved data could no longer start. In that case the cause is not free space on the device but the sheer number of folders the app is holding.
To fix it, open Files → On My iPhone → the app's name and move some folders out of the app — to iCloud Drive or a computer. With less to load, the app can start again, and nothing is lost by moving them.
3. What not to do
Do not use "Offload App".
It looks like a safe way to reinstall while keeping your data, but mistaking it for "Delete App" right below it loses your saved videos and images for good. On top of that it only swaps the app binary — your settings and data stay exactly as they are, so it is very unlikely to fix an app that will not start. All of the risk and none of the benefit, so I do not recommend it.
4. Get your files to safety first
In case the investigation takes a while, it is worth copying your files out now. Even while the app will not start, Files → On My iPhone → the app's name lets you copy everything somewhere else.
5. When you write to me
Since you cannot open the app, please use the contact form. Adding your iOS version, the app version and your device (iPhone 15, iPad, and so on) makes this much quicker to look into. If you know when it started, or which action triggers it, please say so too.