File formate changing

Hello,
just looking for help.don't know how the file format has changed. the file saved in the folder has changed its format to REN file(.ren) and its not able to open with default program.changing the program to open the one particular file it is changing program for all other file in the folder.(example to open the image file by window photo gallery by changing the property it changes all the properties of other file.
plse if anybody can help me to find solution to this problem