open .dat file by another windows

I had a .docx file which I changed it to a .dat file. But after changing Windows, the new Windows does not allow to do any thing with .dat file, e.g. it couldn’t be copied & open (by any application) even if I change the extension to .docx. It sounds the new Windows does not recognize it at all, even when I turn off NOD32. The message by Windows is about encryption and I think Windows finds it as a locked or encrypted file and a kind of permission is needed. Please help me.