What happened and how to fix it? I can, however, use left click button and enter, but it is annoying! Please help me? :)

If you're talking about the left click button being unable to double click, that may be a sensitivity issue that you can adjust under mouse settings. If it's more of a bug/technical problem, then I don't know.
If it's the touchpad that isn't allow you to double click, that may also be something you can fix in the settings.
That's something you can set up in the control panel (windows) or preferences (mac) under the mouse/touchpad settings.

I figured it out. I looked at the lower bottom right portion of my computer, and located the red squared icon that represents touchpad/mouse, I clicked on it and checked off the 'Tap To Click' option and boom there it was, working again :))) Thanks for your effort, my friend! I wish all my problems were this easily taken care of!

Great thanks for the help

@yanagirl, I had the same problem. Your answer really helped me. The 'tap to click' function on my Synaptics pointing device driver had become disabled somehow. I just found it in my mouse settings and then enabled it. Presto! Now its working like before.