I can't join my friend's Minecraft server, despite just playing it yesterday. The IP is correct, my connection is fine, the server is up?

It could be that he hasn't closed his firewall and it's only local host, make sure that other people from locations that aren't near him can also connect.

Check if his or your NAT/Firewall has been set to "Moderate", since setting it to "Strict" were usually the main problem for this kind of situation.

If that doesn't work, then use this as a guide or for troubleshooting