Servers perform a daily restart at 12:15 CEST, it usually takes 10-20 minutes. Sometimes servers go down to upate the game or mods which also needs a restart.
If only one server is offline, it could have been crashed and needs to be restarted by server owners. Please join GD's discord to stay tuned.
If servers are up but you still can't see them, make sure that your client and mods are updated.