You should have gone Folder Options-->View and then untick Hide extensions for known file types.
Without knowing what you've changed, can't help with changing it back.
And to anyone who reads this:
Its a great OS, but I don't think its ready for "basic" peeps like me to use it
. Please don't respond. We have enough threads discussing why it is ready. A quiet dignified silence is the way to go.
Move along, there's nothing to see here.