Ender Dragon Glitch

On my single player Survival mode world, I went into The End, and I saw 1 Ender Dragon. But for some reason, more and more seemed to appear all suddenly. I logged out of my world thinking it was a glitch but when I got back on I died instantly. I lost everything. I have resources to rebuild but it will take a while. If I can get my stuff back that would be great but I’m fine as long as you fix this glitch PLS