If your boiler isn’t firing up, or the timer isn’t working, the first place to check is your fuse box. In most cases the problem is caused by a blown fuse and you can just flick the switch to get yours back on. Some boilers will have their own circuit breaker and this will usually be found in the room where the boiler is.
Most boilers have safety devices built in so if any of these are tripped, then a simple reset may get your whole system working.
If you have a remote control timer or room thermostat, then it can sometimes stop sending signals to the boiler. Check your instructions, and check the batteries too.