Try a Safe Mode boot ...
Startup your Mac in Safe Mode
A Safe Mode boot takes much longer than a normal boot so be patient.
Once you are in Safe Mode, click Restart from the Apple () menu.
Then check available disk space. Click your Apple menu top left in your screen. From the drop down menu click About This Mac > More Info > Storage
Make sure there's at least 15% free disk space.
To fix the Storage display, rebuild the Spotlight index. If you try to search now from the magnifying-glass icon in the top right corner of the display, a pulsing dot will appear in the icon. When the dot disappears, the indexing is complete.