Assuming that you're running on Windows 7:
Make sure you are logged into your account (the account where tap-to-click doesn't work). Go to Start > Control Panel.
In the search box beside the address bar, type "mouse". When the search results show up, click on "Change mouse settings". A window should now pop-up with your trackpad's settings. Navigate to the tab that says "Device Settings" and press the "Settings..." button. Another window should appear. On the list to the left, click the word "Tapping". You should now see a picture of a glowing, green fingerprint on the right side of the window, with a check box that says "Enable Tapping". Check the box, save then exit.