MacBook Pro Trackpad Suddenly Stopped Working Completely
The trackpad is connected over USB - so try these things before seeking hardware service:


Check System Information app like the image below
Disconnect all external devices and restart to reset the NVRAM (that causes the hardware tree to repopulate)
Make a system backup and possibly reinstall the latest combo update / or install a new OS on an external drive to ensure it's not a problem with the OS / software