Cannot go into multiplayer after my game crashed

Helloo!

I was playing custom games with my friend and then we finished playing the game I wanted to switch tabs to my desktop but it became black,
So I clicked on the make another Desktop and closed the game on my second desktop. I went back to my main desktop and tried to open halo and it says an Anti-Cheat warning with some text and after that I restarted my pc just to be sure. When I was back on Steam I launched the game and now whenever i try to go online it says please wait for it to be reconect to our servers but I can play Custom games.

Happened to me before.
Worked again after i restarted my PC.