My 1.7.10 Minecraft server is not working?

I have also had this problem, sometimes the server has been deleted/temporarily removed and yet the ip address still exists, so it won't show up. Other times the server may need to be updated to work properly, or may even be down for maintenance. All I can say is that if you have restarted your game and it still won't work, then there's just a problem with the server, so it's nothing you need to worry about, and as annoying as it is you might have to wait a while for it to be fixed!