Facebook – Is it possible to see all followers of a Facebook account

If you are concerned that you've been hacked and are still able to log in from your laptop, you should change your password immediately.

It is odd that you're unable to do certain things on Facebook. Someone hacking your account shouldn't break basic functionality like viewing replies. If you haven't run a virus scan recently on your machine, give that a shot as well.

First, it seems that what you experienced is no longer happening.

And, as stated in this answer the limit for a message in Facebook Messenger is 20,000 characters, which is true at the moment.

So, your message exceeds the maximum message length supported, and probably led to an unexpected behavior - the error and situation you described.

Nowadays, as from what I tested when trying to send a message that is too long: If the tab didn't got stuck by pasting the message, and you've been able to sent it, the following notice will appear in red under the sent message:

⚠️ These people aren't receiving messages right now.

Which means that the message wasn't actually sent (because of the exceeding length of it) and so it won't be delivered to the other person. If you'll refresh the page you'll see that the message wasn't sent and doesn't exist.