If you feel like you have been hacked then you change your password on Minecraft.net
Secondly I would advise you to read the
rules again. Then not grief ever again, and as you already have two bans for griefing you should know not to do that by now.
Greifing is when you modify/destroy others work without their permission. So DONT do that again as your next ban may be alot longer.
But bare in mind a day ban is a little light for me, so think yourself lucky.