Opening videos on my camera roll

You may try some of these steps, please check after each step...

1. Ensure that the video format is compatible with iOS. The iPhone typically supports H.264 and HEVC (H.265) video codecs. If your video is in an unsupported format, you might need to convert it using a third-party video converter on your computer.
2. Sometimes, a simple restart can resolve temporary glitches. Turn off your iPhone, wait a few seconds, and then turn it back on.
3. Ensure that your iPhone is running the latest version of iOS. Updates often include bug fixes and improvements that may address issues with the Photos app.
   1. Go to "Settings" on your iPhone.
   2. Select "General."
   3. Tap "Software Update" to check for and install any available updates.
4. Insufficient storage space on your iPhone can affect the performance of apps, including the Photos app. Delete any unnecessary photos or videos to free up space.
5. If the videos are stored in iCloud, a stable internet connection is required to stream them. Make sure you have a good Wi-Fi or cellular data connection.
6. Download a third-party video player from the App Store and try playing the videos using that app. If they play successfully, it may indicate an issue with the Photos app.
7. Sometimes, a simple restart can resolve temporary software glitches. Turn off your iPhone, wait for a few seconds, and then turn it back on.
   1. Restart the device. That's it.
   2. Still not solved --> Force restart iPhone - Apple Support
8. If the issue persists, you can try resetting all settings on your iPhone. This will not erase your data but will reset all personalized settings to their default values. Go to Settings > General > Transfer & Reset > Reset > Reset All Settings and confirm the action. Note that you will need to reconfigure your settings afterward. All settings—including network settings, the keyboard dictionary, the Home screen layout, location settings, privacy settings, and Apple Pay cards—are removed or reset to their defaults. No data or media are deleted.
   1. Reset All Settings: Return iPhone settings to their defaults - Apple Support:- Return iPhone settings to their defaults
   2. You can return settings to their defaults without erasing your content.
   3. If you want to save your settings, back up your iPhone before returning them to their defaults. For example, if you’re trying to solve a problem but returning settings to their defaults doesn’t help, you might want to restore your previous settings from a backup.
9. Make sure you have the latest updates installed.
   1. Update Over-the-Air (OTA) —> Update your iPhone or iPad - Apple Support (IN)
   2. Update using iTunes/Finder —> Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If none of the above steps resolve the issue, it is recommended to reach out to Apple Support or visit an authorized service center for further assistance. They can provide specific guidance based on your device model and iOS version.

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.