Help my server pls.

Here's what happened: Yesterday, while I was playing with my friends, a stranger joined our world, but left a few seconds later. I thought he had joined the wrong account, since similar things have happened before on servers that have been running for a long time. But it wasn't that simple. After he left, my friends kept leaving and rejoining, and I thought they were experiencing lag. Then, a second later, I was also removed from the game. The reason seemed to be that someone logged into my account elsewhere. I want to know how to restore the server. I tried backing up the world and testing it in single-player mode, but I couldn't get in. I could only enter the world in "safe mode," and the screen showed a death screen. Could this be caused by some kind of cheat? (This is a translation and might look a bit strange.)