— If the password is known, Recovery for Excel application has the ability to...

— A DAT file can be actually any file. You will have to use the application that...

— To unlock this file you need the password. In case that you lost the password...

— This may have multiple causes like large files, spending time on validation...

— I happened to got this problem and i try to find out any free method to unlock...

— Open the Excel application, go to Options > Advanced > General > Clear...