Have you checked your battery? I had the same problem with mouse pad not working and an Apple technician took a look at it and it was the battery had swollen up so it had pressure on the top. Apple gave me a new battery and the mouse pad works like a charm.

I had replaced my hard drive using directions from ifixit (thanks!). The key point is that the metal piece around the battery compartment was not quite in place right. When I put the battery back in I found that the mouse clicker no longer worked. I discovered it was the battery pressing on the button because the surrounding piece that held it in place was slightly off. Remove, reinstall fully in line and mouse issue solved. :)

Press the ribbon cable under the battery UP against the keyboard.

Have you had the keyboard off? If so you may have moved/broken/disconnected the little cable that goes to the switch.

Good Luck,

N.