How to deal with emergency low boiler water level?

Normally a boiler is provided with two independent sensors for emergency low water level burner cut-outs. So this would never happen. However, if it does, don't take any chances! Shut off the burners immediately! 
Before you start raising the level in the boiler you have to find out if any part of the furnace walls has been overheated. If you raise the level over a glowing steel-wall then the boiler might produce more steam than the safety valves can handle and a nasty explosion would be the result.