https://steamhost.cn/steamcommunity_com/discussions/forum/9/882959527688934488/
https://support.steampowered.com/kb_article.php?ref=7849-RADZ-6869&l=

No one on the forums can remove nor investigate a ban. Only falsely applied VAC bans are removed automatically by the system at a later time.

Known cheats were detected on the computer when the account connected to a vac server. A cheat is any program that alters protected game files, it does not have to give an advantage (skin changers, 3rd party crosshair mods, etc).

You are responsible for the security of the account and what is on the computer(s) the account uses. It does not matter who was using the account to cheat. Bans are not instant, whatever you were doing at the time of the ban being applied is unlikely to be the cause of it.

Steam/Valve will not release any information about what was detected, that information will only help those that cheat and make cheats.

Please read the linked material and click the INFO link next to your ban notice on your profile for more information.