Unless the utility you used to "encrypt" it has a "decrypt" command, you probably need to Restore it.
Select the shuffle in the iTunes sidebar, under DEVICES. Over to the right, go to the Summary or Settings tab (depending on shuffle model). The Restore button is there. This will erase the shuffle, re-install its software, and set it to default settings. Erasing basically reformats the storage, so it should remove the encryption.