Hmm... it might be a possible Minecraft Client issue as
@MLGKillerPro says or maybe you might be using an old IP, Make sure you connect using one of the following IP's
OpBlocks.net
OpBlocks.com
Play.OpBlocks.net
Play.OpBlocks.com
If none of the IP's are working and you tried to restart your Minecraft it might be an issue with your internet. Next time it happens screenshot it and reply with the image. ~JustACookie