In Windows most files have two parts to their name. The first part is the file NAME, the second part the file TYPE.
An example is myholidaypicture.jpg.
"myholidaypicture" is the file name.
"jpg" is the file type (tells Windows it is a picture).
The default in Windows is usually to NOT display the file type (jpg or whatever).
It sounds as though that default has been turned off on your computer.
So you just need to remember if you rename a file only rename the FIRST part and not the SECOND part (jpg or whatever). Just leave the second part as it is.
The reason the file disappears from your list is that Windows thinks it is no longer a picture so will not show it in your list of pictures.
If you can find the file again I suggest you rename it by ADDING a .jpg back on the end (note there is a full stop before the jpg)