I just tried JAilbreakMe on my iPhone and it was on 4.2.1 and i turned off the phone and now it's looping into the apple logo. Please help! DFU doesn't work!!!!!! PLEASE HELP!!!!!!!!!!! I don't want to go to the Apple Store tonight!
That jailbreak does not work on 4.2.1...go download greenpois0n and try jailbreaking it again using their method.
Also, if you just want to reset it so it goes back to the way it was, hold down the power and home button for like 10sec. or so and do a hard restart, should go back to the way it was before you tried the wrong jailbreak for 4.2.1...
(also, there is an entire jailbreak section on this website, for future reference)
Great news! After trying to put the iPhone into DFU mode many many times, it worked but restoring right now to 4.3.3. But I will update this thread if it works.
Now you should focus on trying to make your signature even longer...