Assuming you used a file manager to rename that folder, there is most likely an option in that file manager's Settings to 'show hidden content'. Enable that option and that .nomedia folder should show up so you can rename it back to its original name, without that preceding period of course.
If you need a good, basic file manager take a look at File Expert if you're not rooted. If you are, I really like Root Explorer.
https://play.google.com/store/apps/details?id=xcxin.filexpert&hl=en
https://play.google.com/store/apps/details?id=com.speedsoftware.rootexplorer&hl=en
Just to note, the .nomedia name is typically applied to a blank, no-file-type file that is located
inside a folder, not the folder itself. If you were attempting to flag that folder to be skipped by a media player from indexing its contents, just create a text file (don't add anything to the file itself) inside that folder, rename it .nomedia, and that's it, you don't need to rename the folder itself.