iPhone storage category

Try a forced restart and then see if it shows up. Hold down on the sleep and home buttons at the same time until the Apple logo appears on the screen. Then let go of the buttons and let the phone boot up. After it starts up, see if the storage bar shows up.

Since this a setting that has gone wonky, you can try to reset all settings. It returns the device to out of the box, default settings. No data is lost when you reset settings, but you do have to enter all of the device settings again. That includes having to enter WIFi passwords again.

Return iPhone settings to their defaults - Apple Support

When you reset all settings, the phone will reboot on its own. It takes some time to get everything set to the way you had it, but it just might help.