Make sure Settings > Do Not Disturb is not activated
Make sure you have the most recent iOS version installed
Make sure Settings > Notifications > Calendar > Allow Notifications is activated
Make sure Settings > Notifications > Week Cal > Allow Notifications is activated
Check the volume on the ringer
Go to Settings > Sounds and check to see whether “Ringer and Alerts” are muted. If they are muted, make sure you slide the volume up
Check the mute button. Your device might be frozen in mute mode. Flick the Mute button on and off several times
Disable notifications for Calendar in Settings > Notifications Turn off your phone and turn Settings > Notifications > Calendar back on
Restart your device by pressing and holding the sleep/wake button until you see the slider then slide the slider
Reset your device by holding down the home button and the power button (wake/sleep) until you see the Apple logo
Reset Network Settings. Go to Settings -> General -Reset -> Reset Network Settings Reset All settings Hold the home and lock button together for a few seconds until the iPhone restarts itself
Alerts Not Working - Troubleshooting Steps: