Network Attack

Have your tried turning your wifi router off to begin with and turning it back on, that would normally fix it that being if he or she has your ip address that is.

Talk to your ISP since it could potentially be serious and report it and if you know their user Gamertag or when you get on Xbox Live report them on your computer, mobile phone (website or xbox app) or xbox. Hopefully all goes well.

Your IP address may be exposed on top of this.

I recommend going through these steps attached on the link

https://www.imperva.com/blog/protecting-gamers-from-dos-ddos-attacks/

Hope this helps and once you are connected again I would advise the Player that's specifically attacking you. Best of luck

Since the attack is done through your internet, you'll want to contact your ISP to report the issue. Once you're able to get back on Live, you can report the user then.

First thing you will want to do is hard reset your internet equipment, but leave it unplugged for about ten minutes before plugging it back in.

While that hard resets you should hard reset your xbox. You can hold down the xbox home button and then go right twice to where it says restart console.

Reporting the player and following the guide others have linked will be a good third step.

I’m sorry this happened to you. Hope that helps!