Distributed Denial of Service (DDoS) attacks have been used on the internet for a fair while to target services and bring them offline or generally disrupt them. It happened to Jim Sterling back in 2017 after his review for The Legend of Zelda: Breath of the Wild, and it happens to games like CS:GO as well.
Speaking of which, Valve has spoken at the Game Developer Conference (GDC) about DDoS attacks (as reported by PCGamesN) durign which time Kassidy Gerber said that these attacks "are going to happen" and that they're inevitable for this scale of games. Gerber works within Valve's internet infrastructure departments, and added that developers "need to protect both your players and game servers, because they're both vulnerable to attacks."
This doesn't only happen to Counter-Strike, but Dota 2 as well, and Valve has tools in place to help reroute traffic, hide IP addresses, and more.
What can be done to combat DDoS attacks?