The Minecraft server has been set to “online” mode. As such, your IDs are all going to be different when you sign in. What does that mean to you? Basically, you’re going to have no permissions, your inventory will be missing, and, if you were a member of a faction, you no longer will be.
But don’t despair! It can be easily fixed. I just need to know exactly when you tried to sign in (tell me date and time, as well as your time zone), and I can restore everything back to how it was.
Why was this change made? Well, people had been asking for skins for a very long time, and I finally caved in and did it. Now, using your minecraft.net account, you can choose a custom skin for your character and show your unique style! This does, however, require a minecraft.net account.