corrupted hard drive or bad touch connection.
the touch connection is easy to check... boot it up and look at the green light on the touch controller. when you touch the screen the light should go brighter. if it's flashing (blinking a code), you have a touch overlay issue.
if it's off... the power connection is bad. check where a orange/gray wire pair plug into the 12 volts from the computer power supply.
check the serial cable that plugs in right next to the monitor VGA on the motherboard. just unscrew it, unplug it... then plug it back in again. it could be just a rotten connection.
the other end of that cable where the "microtouch" controller is...if you remove the controller, there is another cable that plugs into the back side. do the same, unplug/ plug back in again. use caution as this connection is easy to mess up or misalign.
then boot it back up again and see if it works. if it seems to work when you touch it (half bright with no touch, full bright with touch), but it still says no touch controller, you might have a bad file on the harddrive and will need to reinstall it or replace it or you have a bad controller.