ender dragon bug

Me and my friend was playing on our server we went to fight the ender dragon and we got it to half health and then it dissapered it hasn't came back since after that we restarted the server and it was on full health and it is not there now we are hoping you can please respond and we could maybe get something in return for not being to beat the game

Thank you for taking your time to have a look into it

Our In game names Clapz99 and Graham_D