The post was deleted
If the other user deleted the post, the notification would be automatically removed from your Facebook.
The person undid an action
If the other person cancels any of their action, then the notification gets deleted. For example, if anyone sends you a friend request, you would get a notification, but if that person cancels the friend request, then the information of the friend request would get deleted from your account.
An issue with your app
In case all the notifications get deleted from your account, there is an issue with your app, and you should report the problem. There can also be a problem with your phone notification settings; therefore, you can check that.
A bug in the app
Sometimes, the Facebook app has bugs that might create a problem with the functioning of the app. Hence, the notifications getting disappeared could be a bug.
You disabled notifications
If you face the notification issue, then you should try and turn notifications on and off for individual apps directly from the phone settings. If you disable the notifications for a particular post, then the notifications associated with it would not appear. (See Why you are suddenly unfriended on Facebook?)
The app needs to be updated
In case if the app is not updated, then the app might not work properly. If you update the app, it might start working correctly, and the issue would get fixed.
If your Facebook notifications do not work, you should delete Facebook and download it again. It would help if you uninstalled it and then install it again, as it can help to fix your issues. (See What happens when you accidentally send a friend request on Facebook)
Restart Facebook on Android or iPhone
A simple restarting of the phone can fix some little glitches. Hence, if you face a notification, you can restart the phone as it might help you.