How can I fix my iPhone alarm after the iOS 18.3 update?

There could be several reasons why your alarms are not sounding or vibrating. Here are some troubleshooting steps you can try:

1. Make sure that the volume is turned up and not on silent or vibrate mode. You can check this by using the volume buttons on the side of your iPhone or by going to Settings > Sounds & Haptics and adjusting the volume slider.
2. Verify that you have selected a sound for your alarms. You can do this by going to the Clock app, selecting the Alarm tab, and tapping "Edit" in the top left corner of the screen. Then, tap on the alarm you want to check and make sure you have chosen a sound.
   1. Probably you have set the Alarm Sound to NONE. Watch the GIF below
   2. 
3. Check for Do Not Disturb or Bedtime Mode: If you have Do Not Disturb or Bedtime Mode enabled, your alarms might be silenced. Check the Control Center or Settings to ensure these features are disabled.
4. 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
5. 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.
6. 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.