U must have had some kind of malicious (Cheat Engine or sth else, even for another game) programm running while playing on an Vacsecured Server. This is no game ban (no Overwatch proceeded), it's an automated Ban, since it was an instant VAC. Else it would have been overwatched first before it got a VAC after a few days. U can claim rights according to EULA if u live in the EU, else I don't know how to help. EULA might helps u and I would try to get a laywer if u are not guilty. I think according to EULA they must tell u which process caused the ban. Hope I could help, good Luck.