Even after a patch aimed at major bugs, Grand Theft Auto 5's Online mode is still going through some growing pains. If the character selection screen shows you empty slots even after you've created a character, Rockstar is warning you not to create a new one in its place.
An update on its support section urges players not to create a new character in an empty slot that previously housed a created character. Instead, it says, you should quit to the story mode or use the pause menu to re-enter GTA Online, and repeat that process until your character shows up again.
It points out that this issue is connected to the cloud servers. When they're down, your character won't appear in the screen but that doesn't mean it's been deleted. Waiting for the cloud to bring your character back should fix the problem, but not if you've overwritten it with a new character.
The post says they are currently working to correct the causes for losses of characters and progress, as well as to find a way to restore lost progress.