Bukkit

I have a minecraft server, using this exact plugin and recently we have discovered an issue. Ive completely set the prices to it the server, and we weren't experiencing any problems, then, out of no where, one of our tabs are completely blank? This is the tab that our whole server practically relies on, Materials. I'm not sure what to do since, one minute it was fine, and the next, blank. Do you know what I have done?