Fireplace will not turn off
It could be a number of different things.  Could be the electronics, could be the gas valve getting stuck open, could be loose/open wire.  You will need to get the manual for the unit and look at it's trouble shooting guide.  If it is not covered in there, time to break out the multi-meter to trace what is going on.  Having a wiring schematic will help with this (should be one in the bay under the unit or in the service manual).