Resistor failed open
If your circuit is built with SMT resistors there is a good possibility that the failure could be due to a fracture of the resistor. 

SMT fractures can be caused by:


Board flexing
Board vibration
Improper board handling
Stress in the component due to bad solder process
Being hit by a tool such as a screwdriver if the resistor is located close to a board mount hole or right next to a screw terminal block where input wires are connected.


If your board is built using THMT (through hole) type resistors then a fractured component could be caused by:


Board vibration
Improper board handling
Stress in the component due using too small of holes in board
Stress in component due to holes being too close together in board or bending the leads too close to the component body.
Being hit by a tool such as a screwdriver if the resistor is located close to a board mount hole or right next to a screw terminal block where input wires are connected.