If you can, access the burner, usually behind a casing door. Is there a red/orange light on at the control box? If there is then the burner has shut down safely because something is wrong. First check to see if you have oil in your fuel tank and that the outlet valve is turned ON. Return to your boiler and press the illuminated lock-out button, the burner should start and run through its start-up procedure. Sometimes you can get a random lock-out that happens for no reason, so pressing the lock-out button may restart the burner and it may run fine again. If your burner goes to lock-out again, then this is the time to get your boiler serviced.