I think this might be a bug or something, but I added my character to a campaign, and my DM was intending to remove it (because it was assumed to have died, he didn't he will be a trick bad guy) however instead of making my character leave the group I lost my character after I told my DM that it was missing he checked his character slots and he had my character. I don't know how but he literally just stole my character, I think this should be fixed to prevent people from getting their character stolen.

Have you tried logging out and logging back in? Maybe it will refresh your account and characters.

Nevermind, someone else reported it as well..

https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/122416-i-stole-a-players-character-somehow

I know almost certainly how this happened.

If a character is unassigned, which it can be done via the DM, and then if a player or DM claims the character it becomes their character, even if they remove the character from the campaign the character becomes theirs. 

Is it weird? Yes. However, if your DM adds the character back to the game, unassigns it and you claim the character it becomes yours again. 

Quote from ConalTheGreat >>

I know almost certainly how this happened.

If a character is unassigned, which it can be done via the DM, and then if a player or DM claims the character it becomes their character, even if they remove the character from the campaign the character becomes theirs. 

Is it weird? Yes. However, if your DM adds the character back to the game, unassigns it and you claim the character it becomes yours again. 

 
This 👆, the DM accidentally reversed the process that is in place for DMs to give characters to players. It’s a design feature. They didn’t “steal it” steal it, they just accidentally reverse gifted it. (If that makes sense.)