In Explorer, go to Tools | Folder Options, and go to the View tab. Then set it to display hidden files. Once that's done, go to the directory your hidden file is in, right click on it, and select properties. In the properties dialog, uncheck the hidden attribute. After that you can go back to the View tab in Folder Options and re-set it to hide hidden files.
If you're comfortable with the command line, you can just go to the directory where the file is and type in attrib -h blah.txt (replace the blah.txt with the name of the file you want to make visible, of course).