When you changed the frequency did you...
1. Turn off the mains supply to the receiver, which means the main supply, not the slider, and also remove the batteries from the thermostat?
2. Press 'reset' afterwards?
3. Check that the switches / jumpers are set the right way around? So, for the jumpers that are missing, the corresponding switches are 'off' down. [Jumper missing = switch set to off]