Could be overpressure switch, underpressure switch, overtemp switch, or, if a closed combustion, a leak in the either the air intake or exhaust or the fan that brings in the outside air may not be pressurizing the system properly. Well, it could be the flame detector, too. In other words, it could be a lot of things, and I probably haven't listed them all. You need to check to see if the control panel has any indicator lights, and review the manual for troubleshooting. A modern boiler has all sorts of safety switches and a sequencer...if something is out of wack, it shuts itself down. It is most likely a sensor switch, but could be lots of things including a real physical problem that the sensor is correctly reporting. If a bad switch, once identified, it may be simple to replace. But, don't just go buying new parts, as all of them can be checked prior to that if you have the proper tools and enough understanding and skill.