Sentinel is a house made Anti-Cheat by
@CrazymanJR, Sentinel doesn't ban instantly but if Sentinel recognise someone with hacks (illegal client modifications) he puts the IGN on a ban list and when the next ban wave is there, because Sentinel bans in ban waves, the player will be banned for 7 days for "Illegal client modifications".
It can also be that someone is falsely banned by Sentinel, it's possible. The only thing you can do and have to do is
appeal and mention what you did (in details) before you got banned, stay nice and mate.