from my searching it doesn't really seem like there's many active servers for zombie survival (the game mode you're looking for, not to be confused with zombie fortress, zombie escape or any other zombie [blank]) but maybe i'm either not looking hard enough, checked at the wrong times, or maybe i'm thoroughly confused!

the other zombie modes still have some activity so.. you can try those..? doesn't have the fun gimmick but it's still zombies at its core. if you're still really dead set on it though i'd advise to just keep checking or idk try and rally some friends/randoms to join a server!

team fortress 2