You might create a profile on a dating website and then receive a message from a user on that dating website.
You might even exchange messages back and forth.
While you are sleeping you receive an email from the dating website that the user has sent you a message.
When you wake up and attempt to read the user's message, the dating website displays "this user does not exist" or something similar.
What happened?
It is possible that the user was chatting with you until he or she found someone he or she decided was more desirable, then he or she blocked you.
While that is certainly a possibility, my best guess would be that what happened is that the dating website banned the user for trying to scam people.
In order to maximize the time until a scammer's profile is banned, a scammer will often try to quickly get potential victims to move communications off the website where you initially met before the scammer introduces his or her scam.