This happened for a number of people yesterday but was resolved for me by last night. At a guess they've tried to roll out some change in the iTunes Store which is either having to be rolled back, or patched further. Try again in a few hours. Once I could hide things again I had to hide around 300 past purchases that I don't want shown because I've tweaked the metadata in some way. Use Feedback - iTunes - Apple or Contact Apple Support if the issue persists.