So lately I've been trying to host a zombie server and I invited my friends to come, but for all of them, my server's ping is ??? no matter what they press whether it be Ping All or Request Server again and again. Is there a reason why this happens? A few days ago I hosted the same game and people were able to join. I don't host dedicated servers. Someone know what to do?

Please use the forum search function, I can't even begin to count how many times this has been asked in the past:

http://archive.forums.soldat.pl/topic.php?topic_id=19977&SearchTerms=ping,???
http://archive.forums.soldat.pl/topic.php?topic_id=23425&SearchTerms=ping,???
http://archive.forums.soldat.pl/topic.php?topic_id=25058&SearchTerms=ping,???
http://soldatforums.com/topic.asp?ARCHIVE=true&TOPIC_ID=16103&SearchTerms=ping,???
http://soldatforums.com/topic.asp?ARCHIVE=true&TOPIC_ID=20145&SearchTerms=ping,???

And there are for sure many more which no longer are kept on the server

Is there a reason why, in the past people were able to join my game? I have never opened any ports in my router before. Why are they not able to join now?

If you read those threads properly, you will find that you have sort of firewall blocking your server. It's up to you to find out what is blocking and how to stop it.