Cold be any one of a bazillion things..
Have you tried rebooting the computer for starters?
Ok.. Between the time that it was working and the time that it stopped working, did you do/change anything? Install any software, modify any system settings, install any updates, let anyone use the computer? and.. what brand/model is it and what version of Windows are you using?
ok, so you should be able to plug-in a regular mouse to use while you figure out the problem...
The trackpad probably has some special software that came with it, or a section in your control panel - have you checked there to make sure it's enabled?
Also, have you looked in your Windows Event Viewer (Start|Control Panel|System and Maintenance| Administrative Tools| Event Viewer.) to see if it's showing any errors there?
this might be a stupid question, but did you check to see if you may have locked your track pad by mistake? ive done that before
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