I got banned today because I had to go AFK to support my sibilings in opening presents, and open a few myself.
A ban almost never results from a single case, it's usually a series of games or an abuse pattern. Besides, in order to get banned you have to pass through the Warning phase and potentially multiple time bans. Did you not receive a Tribunal Reform Card detailing your warning? Otherwise it was likely LeaverBuster, which means you've left numerous games.
I have a xp boost till tommorow in which I wont be able to use becuase of this ban
Riot will not rescind your ban unless you are able to provide sufficient proof through a support ticket that the ban is an error. I'm not sure how XP Boosts are handled with bans, but I know that in the event of a server malfunction the duration is extended. I would assume that the XP Boost does not work that way during a ban and will tick down due to it being a punishment, though don't quote me on that.
My suggestion to you would be to try to work towards leaving less games so this doesn't happen in the future!