You may be experiencing any of these issues:

• You can’t access your phone and a ‘decrypt storage’ message shows up.
• You can access your phone but a ‘decrypt storage’ message pops up.

When the phone is encrypted; accounts, settings, downloaded apps and their data, media, and other files are protected.

To return to an unencrypted phone, you must perform a factory reset. See How to factory data reset a device.

NOTE:  If the phone remains encrypted after a factory data reset, a manual ROM install may be necessary. 

It is also possible to encrypt the files on your storage card. Once you enable storage card encryption, only new files added after encryption will be encrypted. 

Files already on the card will not be encrypted. Storage card encryption doesn't reformat or erase your card. Encrypted files on the storage card can only be opened by the phone that encrypted them.