I literally just fixed mine, i had the same problem for a few days. Try the steps below.
1. Make sure your updated to 1.2.4 or 1.2.5
2. Make sure you've typed in the server address correctly
3. Try to disable your firewall or anti virus if they are combined and see if you can connect
4. Also if your running windows, make sure your windows firewall is disabled
5. If the steps above do not work you may need to add the exceptions into your firewall for javaw.exe ( because minecraft depends on java ) ( google how to do this )
6. If the above steps still do not work then you may need to remove your firewall/antivirus combo application or if they are seperate ( this is what i had to do, I'm running Avast 7 internet security, Also i removed malwarebytes - not sure if that has anything to do with anything but i did it all at once )
7. Once you have done all that back up your minecraft files, -Click on your start orb and type %appdata%, open your roaming folder, and open .minecraft ( vista/7 ). For win xp - click on start menu, click run, type %appdata% and navigate to .minecraft.....Copy your saves and or stats, basically anything you want to backup, then move them to a safe place outside of that folder.
8. Delete .minecraft from %appdata%>roaming, then delete the shortcut if you have one.
9. Assuming you didn't reboot after the removal of your firewall/anti virus, after all have been removed reboot now.
10. Once your back online reinstall minecraft first, navigate to your .minecraft folder after installation and replace your saves/stats or whatever your pulled from that folder previously, open minecraft and try to see if multiplayer works, if it does you know it was a problem with restrictions due to your firewall, reinstall your firewall/virus protection and try minecraft once again, if it does not work see step 5,
Hope these steps worked for you if not sorry these are known fixes, take care.
1. Make sure your updated to 1.2.4 or 1.2.5
2. Make sure you've typed in the server address correctly
3. Try to disable your firewall or anti virus if they are combined and see if you can connect
4. Also if your running windows, make sure your windows firewall is disabled
5. If the steps above do not work you may need to add the exceptions into your firewall for javaw.exe ( because minecraft depends on java ) ( google how to do this )
6. If the above steps still do not work then you may need to remove your firewall/antivirus combo application or if they are seperate ( this is what i had to do, I'm running Avast 7 internet security, Also i removed malwarebytes - not sure if that has anything to do with anything but i did it all at once )
7. Once you have done all that back up your minecraft files, -Click on your start orb and type %appdata%, open your roaming folder, and open .minecraft ( vista/7 ). For win xp - click on start menu, click run, type %appdata% and navigate to .minecraft.....Copy your saves and or stats, basically anything you want to backup, then move them to a safe place outside of that folder.
8. Delete .minecraft from %appdata%>roaming, then delete the shortcut if you have one.
9. Assuming you didn't reboot after the removal of your firewall/anti virus, after all have been removed reboot now.
10. Once your back online reinstall minecraft first, navigate to your .minecraft folder after installation and replace your saves/stats or whatever your pulled from that folder previously, open minecraft and try to see if multiplayer works, if it does you know it was a problem with restrictions due to your firewall, reinstall your firewall/virus protection and try minecraft once again, if it does not work see step 5,
Hope these steps worked for you if not sorry these are known fixes, take care.