Can't join specific server.

I can’t connect to my friends Garrysmod server…I get a “connection failed after 4 retries” error.
I could connect like 2 days ago, and every other Garrysmod server works fine.
I know the server is up and running completly fine because there is 10 other people online, and some of my friends are playing on there now!
It’s not blacklisted or anything, and i have tried restarting my computer…
Please help me!

Edit: I just logged in with a proxy (i hope thats legal on here) and it works fine…When i shut off the proxy (VPN proxy) it stops working again.
Note: i am not banned from the server.

If the hosting provider has blocked your IP, you will not be able to see the server at all in the server list since you won’t be having access to even query the server. This is probably the reason. Tell your friend to talk to his hoster and see if they have your IP on the blacklist on their firewall.

If he hosts from home, his router has probably blocked you automatically and it might be hard to unblock you depending on his router firmware and product.

If you’ve tried anything suspicious like DDoSing the server, you’d probably get blocked automatically by the hoster or hus router.