You're probaly speaking about http://minecraftpvp.com/?
The servers they run uses a mod, which in some situations ban people for 24 hour because they cheat. But sometimes this mod fails at banning and might ban you for no reason.
I suggest you wait it out.
You can also research yourself on the forums: http://forum.minecraftpvp.com/yaf_topics56_Banned-from-the-servers.aspx