I guess the files will not disappear from the folder. They will still remain in the concerned folder, but will not be accessed by the application.
For instance, if you rename a .jpg file and rename it by omitting the extension, the file will still remain in the folder itself, but cannot be opened by any of the apps (be it gallery or any other image apps) because the application will not be able to identify the file format.
I am not sure of the way you have been using to rename a file on your Android, but the correct way of doing so should be as follows -
1. Go to your app drawer.
2. Locate and launch the My Files if you are using a Samsung device, or the inbuilt file manager on your device.
3. Navigate to the folder containing the file you want to rename.
4. Open the folder and locate the file you want to rename.
5. Long press on the file to access the shortcut menu. Locate and tap on Rename option.
6. Type in the name you would wish. Be sure you are not changing the file extension.
Done! That should do it.
If you feel.you have lost a renamed file, use the same file manager to search all your folders and files one by one. The files will not disappear unless you delete them.
If you are renaming the files on your PC, please make sure you are not changing or removing the file extension.