We don’t know. Really, we don’t. Only the person who originally sent the file knows. When you try to open the file, a window appears asking you for the password. Toward the bottom of that window is the name of the person who originally sent the file and who created the password. If you know that person, contact them. If you don’t, contact the person who sent you the file.
If you need some time to get the password, just click the Cancel button. The file will remain in your received list with a download button.