A common boiler issue people face is when a home has hot water but no central heating. This can happen because of several reasons.
Firstly, if you have a combi boiler, it might accidentally be set to only heat water but not the central heating. This can happen if the boiler is set to this mode during the summer months when radiators do not need to be heated but is forgotten about. It may also be that the boiler is set to turn on at an unexpected time. Check the boiler display to see if this is the case.
Another cause could be that the boiler is working fine but there is trapped air in the radiators stopping them from heating up. The solution there is to bleed the radiators to allow the trapped air to leave the system.
If your central heating system is still not turning on, despite having hot water, it may be that the boiler or heating system is faulty. A Gas Safe registered engineer will be able to investigate the fault and find the source of the issue.