How to turn off the disk mode?

Try Resetting your iPod. Toggle the hold switch off and on a few times then hold down the MENU and SELECT (centre) buttons for five or ten seconds until you see the Apple logo appear then the menu button to go to the normal display: How to reset an iPod

Keep trying, sometimes you have to persevere to get the iPod to reset. Did you read this tip in the how to reset page?

Tip: If you are having difficulty resetting your iPod, set it on a flat surface. Make sure the finger pressing the Select button is not touching any part of the click wheel. Also make sure that you are pressing the Menu button toward the outside of the click wheel, and not near the center.

If the above steps did not work, try connecting iPod to a power adapter and plug the power adapter into an electrical outlet, or connect iPod to your computer. Make sure the computer is turned on and isn't set to go to sleep.