Can you please tell us which Calendar app are you using in your Galaxy S24?
If your manual input events are missing from your Samsung calendar on your Galaxy S24, there are several steps you can take to recover them:
Please ensure you're viewing the correct calendar account: Open the Samsung Calendar app. Tap the Menu icon (three lines in the top left corner). Ensure that the calendar where you saved the events is selected. If your desired account isn’t listed, add it via Settings > Accounts and Backup > Manage Accounts > Add Account.
Sometimes, sync issues can cause events to disappear: Go to Settings > Accounts and Backup > Manage Accounts. Select your calendar account (e.g., Google or Samsung).
Ensure Sync Calendar is turned on. Open the Calendar app and refresh it by pulling down from the top.
If you're using Google's Calendar, and Gmail, these two apps are linked to each other, but I can't see any reason why your manually entered Calendar events would have disappeared.
As @Helenna suggested, making sure that you have the correct Google / Samsung Account selected, if you have more than one email account, would be the most likely reason why you don't see your manual entries.
It's definitely not a problem with the device's firmware, or hardware, so there has to be something else that is preventing your device from syncing your manual entries to your calendar.
You could try Booting into Safe Mode, and see if you can add a Calendar entry.. although I'm not 100% sure if Safe Mode would disable it..
But definitely worth a try.
If you're using any package disabler, or VPN, turn them off before you try.
