All of a sudden when I open a document it opens in notepad and i cannot read it. it is symbols and numbers why?

I tried to open a document that was sent to me and received a message that it might not be encoded correctly. I thought that if I looked under ';encoding'; I could figure it out. I must have hit something to make it change because now when I open a document, it automatically opens it in notepad and is just symbols and numbers. It has never done this before. What did I do?All of a sudden when I open a document it opens in notepad and i cannot read it. it is symbols and numbers why?
Right click on the file, choose open with, make defualt choice, choose whatever you want it to open with and it should work.