Use Arduino to detect momentary 120VAC interruption
Get a 5V transformer, put a bridge rectifier on it, connect it to a digital input, add a 10k resistor in series and a 5V zener diode across to protect the input from overvoltage as transformer will probably give you greater than 5V peaks. 

In arduino firmware set up an external interrupt on said digital input. You should be getting interrupts 100 or 120 times per second. On every interrupt record a timestamp. In the main loop check if last recorded timestamp is older than 0.1 second.