There are many reasons a boiler many not work, including no power or low water level, so fixing the boiler is dependent upon the issues. A boiler many not work because of a malfunction to the electronic burner or pilot, or a problem with the thermostat.
If the boiler is not working because of lack of power, it may be because the breaker is tripped or the fuse that controls the boiler is blown. To repair those issues, reset the tripped circuit breaker or replace the fuse.
If a boiler isn't working because the water is too low, maintain the boiler's water level at half full. The boiler should have a filling system that uses a pressure release valve in order to maintain water levels. If the boiler does not have such a valve, open the water feed valve and manually feed the unit until it reaches 12 pounds per square inch.
If the pilot light is out, relight the standing pilot. If the boiler does not have a standing pilot, troubleshoot the electronic ignition. If the boiler doesn't work because of the thermostat, check that the thermostat is set in heat mode and that it's at an appropriate temperature setting.