Heating thermostat help!

Hi - my heating controls have come away from the main box on the wall and I can't seem to put it back. It may be something as simple as being able to slot it back on but it's possible that it might be broken and I need to buy new controls. It's really annoying as it obviously controls the heating and I can't switch it on without it. My boiler is working fine. I need someone to come and take a look. I don't really want to be charged a massive call-out if it is something that will take a couple of minutes. If anyone is able to recommend a professional who is able to help with this I would be really grateful. Also where is the best place to buy heating controls should I need to replace the old one?