Alright, i was having this problem earlier on and can't access to the server. After searching around the internet, i found a solution which helped me to solve this problem, however i can't guarantee you that this will help you or whatsoever.
AND I WILL NOT BE RESPONSIBLE FOR WHATEVER HAPPENS TO YOUR COMPUTER OR WHATSOEVER. O.O
Vista users.
Firstly go to your desktop and follow with the picture below.

After that, this will pop out and as usual, follow with the picture below.

After clicking, you will be shown another pop out, disable all connections containing "Microsoft Loopback Adapter"

XP users.
Follow the steps of this link below,
http://www.rasyid.net/2007/06/07/uninstallling-remove-microsoft-loopback-adapter-in-windows-xp/Window 7's users.
Firstly go to your desktop and follow with the picture below.

For Windows 7 users, in step 2, click on "Change adapter settings" instead of "Manage Network Connections"
After that, this will pop out and as usual, Click "Change Adapter Settings" instead of "Manage Network Connections"

After clicking, you will be shown another pop out, disable all connections containing "Microsoft Loopback Adapter"

One of the reasons that the Looback Adapter is on, might be possibly because you played one of v88+ servers. Their clients will enable/install the Loopback Adapter automactically each time you click their client!
Lastly i hope this thread will help you solve your problem. However, check with your friends if the server is running smoothly first (server might be experiencing problems)* before attempting this.
LOVE amber