If you're one of the 50 million accounts upgraded to Windows Live Hotmail Wave 4, chances are you must have used "Conversation View" —the features has been now been temporarily disabledt. MVP Vasudev tweeted that "'Conversation View' in the Windows Live Hotmail Wave 4 Beta has been temporarily disabled. #WinLive."
You can check it by going to "Customize your Mail>Conversation Settings," which simply isn't there at the moment, as per instructions on Windows Live Help to enable Conversation Threading:
Update: Conversation View features is back now.