To repair radiant heat systems, find the manual for the unit to find where the power and heating wires connect to the thermostat. Once located, use a multimeter to test the voltage to the thermostat. Another idea is to turn off the heat and disconnect the power and heating leads from the thermostat. Check the wire and insulation resistance numbers compared to the manual, and if they are incongruous, use an arc of voltage to disrupt breaks in the wire.
If the arc of voltage is not successful, repeat the process until the floor is warm enough to pick up on a thermal imaging camera. Find the place where the heat has not showing up on the image, and open up the floor at that spot. Use a wire repair kit to splice the malfunctioning wire to remove the problematic area.
If the numbers are right, find any heating coil disruptions. Use a thermal imaging camera on the floor when the heat is on to check for any disruptions. This includes some coils warming while others do not or some coils overheating. If there is a disruption, open up the floor in the appropriate area, and find the wiring around the malfunctioning coils. Use the repair kit to splice bad wiring. If the problem is because of insulation damage, use shrink tubing from the repair kit to replace the sheath of the wire.