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Hi Sir, Please restart your phone and check it once. Sometimes our phone holds garbage memory if it is not restarted for a long time. If it does not work please let me know. If works Please accept the answer.
Looks like your memory card has been corrupted. It happens to the desktop machine as well where it shows you 500 GB free but when you download something, It does not get downloaded because of hard disk corruption. You need to show your phone to your local vendor. It is an hardware problem. You will be given a new phone if it is in warranty.