You may want to try checking the phone for Viruses and Malware. Those would take up a lot of space. Should that not help, you can clear the caches of apps you do have on the phone. If that still doesn't work, as a last resort you can do a "Hard Reset" of the device. However, I would really make sure you back up all your personal data (photos, videos, contacts, music, etc.) to a computer or cloud account prior to doing this, as it will wipe all data on the device.
NOTE: Doing this will clear 'all' the data on your phone, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the phone.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the...