To help us with a speedy remedy we would really appreciate you providing us as much detail as possible including photos of the control panel (where applicable). Many of these issues can be fixed over the phone rather than sending out a contractor. The more details the better please. 

Is the problem with all rooms? a single room? 
Is the problem with hot water only? radiators only? all heat to the property?
Have you changed anything recently that may have caused this? 
What is the temperature in the property? What would you like it to be?

Common Problems and Solutions
Bleeding Your Radiators - This is a tenants responsibility and a rather simple job and can mean the difference between a hot or cold room. 

We suggest you read these articles on bleeding a Radiator - and 
This will save your the cost of a call out fee to your property. If you have bled the radiator and its still not working them please register an issue.